Make RebaseCommands.AmendTo more robust

This fixes two problems with the "amend commit with staged changes" command:

1. Amending to a fixup commit didn't work (this would create a commmit with the
   title "fixup! fixup! original title" and keep that at the top of the branch)
2. Unrelated fixup commits would be squashed too.

The added integration test verifies that both of these problems are fixed.

1 files changed, 14 insertions, 2 deletions

diff --git a/pkg/commands/git_commands/rebase.go b/pkg/commands/git_commands/rebase.go
index be5384773..50c599a66 100644
--- a/pkg/commands/git_commands/rebase.go
+++ b/pkg/commands/git_commands/rebase.go
@@ -214,12 +214,24 @@ func (self *RebaseCommands) PrepareInteractiveRebaseCommand(opts PrepareInteract
 }
 
 // AmendTo amends the given commit with whatever files are staged
-func (self *RebaseCommands) AmendTo(commit *models.Commit) error {
+func (self *RebaseCommands) AmendTo(commits []*models.Commit, commitIndex int) error {
+	commit := commits[commitIndex]
+
 	if err := self.commit.CreateFixupCommit(commit.Sha); err != nil {
 		return err
 	}
 
-	return self.SquashAllAboveFixupCommits(commit)
+	// Get the sha of the commit we just created
+	fixupSha, err := self.cmd.New("git rev-parse --verify HEAD").RunWithOutput()
+	if err != nil {
+		return err
+	}
+
+	return self.PrepareInteractiveRebaseCommand(PrepareInteractiveRebaseCommandOpts{
+		baseShaOrRoot:  getBaseShaOrRoot(commits, commitIndex+1),
+		overrideEditor: true,
+		instruction:    daemon.NewMoveFixupCommitDownInstruction(commit.Sha, fixupSha),
+	}).Run()
 }
 
 // EditRebaseTodo sets the action for a given rebase commit in the git-rebase-todo file