Show GPG error before entering commit editor when rewording non-latest commits (#4660)

- **PR Description**

When GPG signing is enabled but overrideGpg is false, attempting to
reword a non-latest commit would show the "Feature not vailable for
users using GPG" error only after the user had already entered the
commit message.

Now the error is displayed immediately when attempting to start the
reword operation, before opening the editor.

Closes #4611

pkg/commands/git_commands/rebase.go

@@ -35,6 +35,8 @@ func NewRebaseCommands(
 }
 
 func (self *RebaseCommands) RewordCommit(commits []*models.Commit, index int, summary string, description string) error {
+	// This check is currently unreachable (handled in LocalCommitsController.reword),
+	// but kept as a safeguard in case this method is used elsewhere.
 	if self.config.NeedsGpgSubprocessForCommit() {
 		return errors.New(self.Tr.DisabledForGPG)
 	}

pkg/gui/controllers/local_commits_controller.go

@@ -343,6 +343,10 @@ func (self *LocalCommitsController) fixup(selectedCommits []*models.Commit, star
 }
 
 func (self *LocalCommitsController) reword(commit *models.Commit) error {
+	commitIdx := self.context().GetSelectedLineIdx()
+	if self.c.Git().Config.NeedsGpgSubprocessForCommit() && !self.isHeadCommit(commitIdx) {
+		return errors.New(self.c.Tr.DisabledForGPG)
+	}
 	commitMessage, err := self.c.Git().Commit.GetCommitMessage(commit.Hash())
 	if err != nil {
 		return err
@@ -352,7 +356,7 @@ func (self *LocalCommitsController) reword(commit *models.Commit) error {
 	}
 	self.c.Helpers().Commits.OpenCommitMessagePanel(
 		&helpers.OpenCommitMessagePanelOpts{
-			CommitIndex:      self.context().GetSelectedLineIdx(),
+			CommitIndex:      commitIdx,
 			InitialMessage:   commitMessage,
 			SummaryTitle:     self.c.Tr.Actions.RewordCommit,
 			DescriptionTitle: self.c.Tr.CommitDescriptionTitle,