Use 'break' instead of 'edit' for BeginInteractiveRebaseForCommit with merge commit

BeginInteractiveRebaseForCommit is used for all the patch commands, and for
rewording. It works by setting the commit we want to stop at to 'edit'; this
doesn't work for merge commits. This wasn't a problem for the patch commands so
far, because you typically don't use custom patches with merge commits (although
we don't prevent this; maybe we should?).

However, it was a problem when you tried to reword a merge commit; this
previously failed with an error, as the test added in the previous commit
demonstrated.

Also, we want to add a new patch command that has to stop *before* the selected
commit (pull patch to new commit before the original one), and this wouldn't
work for the first commit in a feature branch, because it would have to set the
last commit before that to 'edit', which isn't possible if that's a merge (which
is likely).

To fix all this, use a 'break' before the selected commit if the commit is a
merge. It is important that we only do it in that case and not always, otherwise
we would break the new regression tests that were added a few commits ago.

Author: stefanhaller

pkg/commands/git_commands/rebase.go

@@ -400,7 +400,19 @@ func (self *RebaseCommands) SquashAllAboveFixupCommits(commit *models.Commit) er
 func (self *RebaseCommands) BeginInteractiveRebaseForCommit(
 	commits []*models.Commit, commitIndex int, keepCommitsThatBecomeEmpty bool,
 ) error {
-	return self.BeginInteractiveRebaseForCommitRange(commits, commitIndex, commitIndex, keepCommitsThatBecomeEmpty)
+	if commitIndex < len(commits) && commits[commitIndex].IsMerge() {
+		if self.config.NeedsGpgSubprocessForCommit() {
+			return errors.New(self.Tr.DisabledForGPG)
+		}
+
+		return self.PrepareInteractiveRebaseCommand(PrepareInteractiveRebaseCommandOpts{
+			baseHashOrRoot:             getBaseHashOrRoot(commits, commitIndex),
+			instruction:                daemon.NewInsertBreakInstruction(),
+			keepCommitsThatBecomeEmpty: keepCommitsThatBecomeEmpty,
+		}).Run()
+	} else {
+		return self.BeginInteractiveRebaseForCommitRange(commits, commitIndex, commitIndex, keepCommitsThatBecomeEmpty)
+	}
 }
 
 func (self *RebaseCommands) BeginInteractiveRebaseForCommitRange(

pkg/integration/tests/interactive_rebase/reword_merge_commit.go

@@ -40,17 +40,11 @@ var RewordMergeCommit = NewIntegrationTest(NewIntegrationTestArgs{
 					Type("renamed merge").
 					Confirm()
 			}).
-			/* EXPECTED:
 			Lines(
 				Contains("CI ◯ two"),
 				Contains("CI ⏣─╮ renamed merge").IsSelected(),
 				Contains("CI │ ◯ one"),
 				Contains("CI ◯ ╯ base"),
 			)
-			ACTUAL: */
-			Tap(func() {
-				t.ExpectPopup().Alert().Title(Equals("Error")).
-					Content(Contains("error: 'edit' does not accept merge commits"))
-			})
 	},
 })