Fix an issue where in some cases 'wrong' commits are shown in listed branches

This was because the merge base computed was wrong. Instead of relying on the default target.sha, which may have become stale, read the git reference of the default target

Author: krlvi

crates/gitbutler-stack/src/stack.rs

@@ -310,9 +310,15 @@ impl Stack {
     /// - If a target is not set for the project
     /// - If the head commit of the stack is not found
     pub fn merge_base(&self, stack_context: &StackContext) -> Result<git2::Oid> {
-        let target = stack_context.target();
-        let repository = stack_context.repository();
-        let merge_base = repository.merge_base(self.head(&repository.to_gix()?)?, target.sha)?;
+        let gix_repo = stack_context.repository().to_gix()?;
+        let target_sha = gix_repo
+            .find_reference(&stack_context.target().branch.to_string())?
+            .peel_to_commit()?
+            .id
+            .to_git2();
+        let merge_base = stack_context
+            .repository()
+            .merge_base(self.head(&gix_repo)?, target_sha)?;
         Ok(merge_base)
     }