Add three new goals that involve keeping the border commits

README.rst

@@ -195,8 +195,106 @@ history by using either the ``finish`` or ``simplify`` command.  The
     approach.  See [2]_ for more information.
 
 ``full``
+
     don't simplify the incremental merge at all: do all of the
     intermediate merges and retain them all in the permanent history.
+    In other words, it transforms this::
+
+        o---o---1---2---3      BRANCH1
+             \
+              A---B---C---D    BRANCH2
+
+    into this::
+
+        o---o---1---2---3
+             \   \   \   \
+              A---A1--A2--A3
+               \   \   \   \
+                B---B1--B2--B3
+                 \   \   \   \
+                  C---C1--C2--C3
+                   \   \   \   \
+                    D---D1--D2--D3    NEW_BRANCH
+
+    This approach retains the complete history and ancestry
+    information, which gives the maximum flexibility for conducting
+    future merges. On the other hand, it clutters up the permanent Git
+    history considerably.
+
+``border``
+    this experimental goal retains the rebase of ``BRANCH2`` onto
+    ``BRANCH1`` and also the rebase of ``BRANCH1`` onto ``BRANCH2``,
+    plus a merge commit that includes both branches. In other words,
+    it transforms this::
+
+        o---o---1---2---3      BRANCH1
+             \
+              A---B---C---D    BRANCH2
+
+    into this::
+
+        o---o---1---2---3
+             \           \
+              A           A2
+               \           \
+                B           B2
+                 \           \
+                  C           C2
+                   \           \
+                    D---D1--D2--D3    NEW_BRANCH
+
+    This approach leaves more history than a simple merge or rebase,
+    possibly making future merges easier.
+
+``border-with-history``
+    this experimental goal retains the rebase-with-history of
+    ``BRANCH2`` onto ``BRANCH1`` and also the rebase (without history)
+    of ``BRANCH1`` onto ``BRANCH2``, plus a merge commit that includes
+    both branches. In other words, it transforms this::
+
+        o---o---1---2---3      BRANCH1
+             \
+              A---B---C---D    BRANCH2
+
+    into this::
+
+        o---o---1---2---3
+             \           \
+              A-----------A3
+               \           \
+                B-----------B3
+                 \           \
+                  C-----------C3
+                   \           \
+                    D---D1--D2--D3    NEW_BRANCH
+
+    This approach leaves more history and ancestry information than a
+    simple merge or rebase, possibly making future merges easier.
+
+``border-with-history2``
+    this experimental goal retains the rebase-with-history of
+    ``BRANCH1`` onto ``BRANCH2`` and also the rebase-with-history of
+    ``BRANCH2`` onto ``BRANCH1``, plus a merge commit that includes
+    both branches. In other words, it transforms this::
+
+        o---o---1---2---3      BRANCH1
+             \
+              A---B---C---D    BRANCH2
+
+    into this::
+
+        o---o---1---2---3
+             \   \   \   \
+              A--- --- ---A3
+               \   \   \   \
+                B--- --- ---B3
+                 \   \   \   \
+                  C--- --- ---C3
+                   \   \   \   \
+                    D---D1--D2--D3    NEW_BRANCH
+
+    This approach leaves more history and ancestry information than a
+    simple merge or rebase, possibly making future merges easier.
 
 
 Technical notes

git-imerge

@@ -122,8 +122,11 @@ ZEROS = '0' * 40
 
 ALLOWED_GOALS = [
     'full',
-    'rebase-with-history',
     'rebase',
+    'rebase-with-history',
+    'border',
+    'border-with-history',
+    'border-with-history2',
     'merge',
     'drop',
     'revert',
@@ -2864,6 +2867,86 @@ class MergeState(Block):
 
         self._set_refname(refname, commit, force=force)
 
+    def simplify_to_border(
+            self, refname,
+            with_history1=False, with_history2=False, force=False,
+            ):
+        i1 = self.len1 - 1
+        for i2 in range(1, self.len2):
+            if not (i1, i2) in self:
+                raise Failure(
+                    'Cannot simplify to border because '
+                    'merge %d-%d is not yet done'
+                    % (i1, i2)
+                    )
+
+        i2 = self.len2 - 1
+        for i1 in range(1, self.len1):
+            if not (i1, i2) in self:
+                raise Failure(
+                    'Cannot simplify to border because '
+                    'merge %d-%d is not yet done'
+                    % (i1, i2)
+                    )
+
+        i1 = self.len1 - 1
+        commit = self[i1, 0].sha1
+        for i2 in range(1, self.len2 - 1):
+            orig = self[0, i2].sha1
+            tree = self.git.get_tree(self[i1, i2].sha1)
+
+            # Create a commit, copying the old log message:
+            if with_history2:
+                parents = [commit, orig]
+                msg = (
+                    self.git.get_log_message(orig).rstrip('\n')
+                    + '\n\n(rebased-with-history from commit %s)\n' % (orig,)
+                    )
+            else:
+                parents = [commit]
+                msg = (
+                    self.git.get_log_message(orig).rstrip('\n')
+                    + '\n\n(rebased from commit %s)\n' % (orig,)
+                    )
+
+            commit = self.git.commit_tree(tree, parents, msg=msg)
+        commit1 = commit
+
+        i2 = self.len2 - 1
+        commit = self[0, i2].sha1
+        for i1 in range(1, self.len1 - 1):
+            orig = self[i1, 0].sha1
+            tree = self.git.get_tree(self[i1, i2].sha1)
+
+            # Create a commit, copying the old log message:
+            if with_history1:
+                parents = [orig, commit]
+                msg = (
+                    self.git.get_log_message(orig).rstrip('\n')
+                    + '\n\n(rebased-with-history from commit %s)\n' % (orig,)
+                    )
+            else:
+                parents = [commit]
+                msg = (
+                    self.git.get_log_message(orig).rstrip('\n')
+                    + '\n\n(rebased from commit %s)\n' % (orig,)
+                    )
+
+            commit = self.git.commit_tree(tree, parents, msg=msg)
+        commit2 = commit
+
+        # Construct the apex commit:
+        tree = self.git.get_tree(self[-1, -1].sha1)
+        msg = (
+            'Merge %s into %s (using imerge border)'
+            % (self.tip2, self.tip1)
+            )
+
+        commit = self.git.commit_tree(tree, [commit1, commit2], msg=msg)
+
+        # Update the reference:
+        self._set_refname(refname, commit, force=force)
+
     def _simplify_to_path(self, refname, base, path, force=False):
         """Simplify based on path and set refname to the result.
 
@@ -2976,10 +3059,18 @@ class MergeState(Block):
 
         if self.goal == 'full':
             self.simplify_to_full(refname, force=force)
-        elif self.goal == 'rebase-with-history':
-            self.simplify_to_rebase_with_history(refname, force=force)
         elif self.goal == 'rebase':
             self.simplify_to_rebase(refname, force=force)
+        elif self.goal == 'rebase-with-history':
+            self.simplify_to_rebase_with_history(refname, force=force)
+        elif self.goal == 'border':
+            self.simplify_to_border(refname, force=force)
+        elif self.goal == 'border-with-history':
+            self.simplify_to_border(refname, with_history2=True, force=force)
+        elif self.goal == 'border-with-history2':
+            self.simplify_to_border(
+                refname, with_history1=True, with_history2=True, force=force,
+                )
         elif self.goal == 'drop':
             self.simplify_to_drop(refname, force=force)
         elif self.goal == 'revert':

t/test-conflicted

@@ -14,7 +14,10 @@ cd "$TMP"
 # Clean up detritus from possible previous runs of this test:
 git checkout master
 "$GIT_IMERGE" remove --name=c-d || true
-for b in c-d-merge c-d-rebase c-d-rebase-with-history c-d-full
+for b in c-d-merge \
+	     c-d-rebase c-d-rebase-with-history \
+	     c-d-border c-d-border-with-history c-d-border-with-history2 \
+	     c-d-full
 do
     git branch -D $b || true
 done
@@ -37,6 +40,9 @@ git add conflict.txt
 GIT_EDITOR=cat "$GIT_IMERGE" simplify --goal=merge --branch=c-d-merge
 "$GIT_IMERGE" simplify --goal=rebase --branch=c-d-rebase
 "$GIT_IMERGE" simplify --goal=rebase-with-history --branch=c-d-rebase-with-history
+"$GIT_IMERGE" simplify --goal=border --branch=c-d-border
+"$GIT_IMERGE" simplify --goal=border-with-history --branch=c-d-border-with-history
+"$GIT_IMERGE" simplify --goal=border-with-history2 --branch=c-d-border-with-history2
 "$GIT_IMERGE" remove
 
 git checkout c

t/test-unconflicted

@@ -14,7 +14,10 @@ cd "$TMP"
 # Clean up detritus from possible previous runs:
 git checkout master
 "$GIT_IMERGE" remove --name=a-b || true
-for b in a-b-merge a-b-rebase a-b-rebase-with-history a-b-full
+for b in a-b-merge \
+	     a-b-rebase a-b-rebase-with-history \
+	     a-b-border a-b-border-with-history a-b-border-with-history2 \
+	     a-b-full
 do
     git branch -D $b || true
 done
@@ -26,6 +29,9 @@ git checkout a
 GIT_EDITOR=cat "$GIT_IMERGE" simplify --goal=merge --branch=a-b-merge
 "$GIT_IMERGE" simplify --goal=rebase --branch=a-b-rebase
 "$GIT_IMERGE" simplify --goal=rebase-with-history --branch=a-b-rebase-with-history
+"$GIT_IMERGE" simplify --goal=border --branch=a-b-border
+"$GIT_IMERGE" simplify --goal=border-with-history --branch=a-b-border-with-history
+"$GIT_IMERGE" simplify --goal=border-with-history2 --branch=a-b-border-with-history2
 "$GIT_IMERGE" remove
 
 git checkout a