[test_helper.rb] Add TestRepo#checkout_b

A instance method helper for TestRepo which performs a checkout out of a
new branch based off another (or the current branch if no second arg is
passed).

Mirrors the usage of `git checkout -b [new_branch] [base_branch]`

Author: NickLaMuro

test/test_helper.rb

@@ -223,6 +223,12 @@ def checkout branch
       repo.checkout branch
     end
 
+    def checkout_b branch, source = nil
+      repo.create_branch(*[branch, source].compact)
+      repo.checkout branch
+      @last_commit = repo.last_commit
+    end
+
     # Commit with all changes added to the index
     #
     #   $ git add . && git commit -am "${msg}"