Ensure committable files for "amend to" and "create fixup commit"

These would previously fail with confusing error messages when no files were
staged.

The diff is best viewed with "ignore whitespace" turned on.

pkg/gui/controllers/local_commits_controller.go

@@ -568,10 +568,12 @@ func (self *LocalCommitsController) moveUp(commit *models.Commit) error {
 
 func (self *LocalCommitsController) amendTo(commit *models.Commit) error {
 	if self.isHeadCommit() {
-		if err := self.c.Helpers().AmendHelper.AmendHead(); err != nil {
-			return err
-		}
-		return self.c.Refresh(types.RefreshOptions{Mode: types.ASYNC})
+		return self.c.Helpers().WorkingTree.WithEnsureCommitableFiles(func() error {
+			if err := self.c.Helpers().AmendHelper.AmendHead(); err != nil {
+				return err
+			}
+			return self.c.Refresh(types.RefreshOptions{Mode: types.ASYNC})
+		})
 	}
 
 	if self.c.Git().Status.WorkingTreeState() != enums.REBASE_MODE_NONE {
@@ -582,10 +584,12 @@ func (self *LocalCommitsController) amendTo(commit *models.Commit) error {
 		Title:  self.c.Tr.AmendCommitTitle,
 		Prompt: self.c.Tr.AmendCommitPrompt,
 		HandleConfirm: func() error {
-			return self.c.WithWaitingStatus(self.c.Tr.AmendingStatus, func(gocui.Task) error {
-				self.c.LogAction(self.c.Tr.Actions.AmendCommit)
-				err := self.c.Git().Rebase.AmendTo(self.c.Model().Commits, self.context().GetView().SelectedLineIdx())
-				return self.c.Helpers().MergeAndRebase.CheckMergeOrRebase(err)
+			return self.c.Helpers().WorkingTree.WithEnsureCommitableFiles(func() error {
+				return self.c.WithWaitingStatus(self.c.Tr.AmendingStatus, func(gocui.Task) error {
+					self.c.LogAction(self.c.Tr.Actions.AmendCommit)
+					err := self.c.Git().Rebase.AmendTo(self.c.Model().Commits, self.context().GetView().SelectedLineIdx())
+					return self.c.Helpers().MergeAndRebase.CheckMergeOrRebase(err)
+				})
 			})
 		},
 	})
@@ -709,12 +713,14 @@ func (self *LocalCommitsController) createFixupCommit(commit *models.Commit) err
 		Title:  self.c.Tr.CreateFixupCommit,
 		Prompt: prompt,
 		HandleConfirm: func() error {
-			self.c.LogAction(self.c.Tr.Actions.CreateFixupCommit)
-			if err := self.c.Git().Commit.CreateFixupCommit(commit.Sha); err != nil {
-				return self.c.Error(err)
-			}
+			return self.c.Helpers().WorkingTree.WithEnsureCommitableFiles(func() error {
+				self.c.LogAction(self.c.Tr.Actions.CreateFixupCommit)
+				if err := self.c.Git().Commit.CreateFixupCommit(commit.Sha); err != nil {
+					return self.c.Error(err)
+				}
 
-			return self.c.Refresh(types.RefreshOptions{Mode: types.ASYNC})
+				return self.c.Refresh(types.RefreshOptions{Mode: types.ASYNC})
+			})
 		},
 	})
 }