fixup! Add commit menu entry "Add co-author"

No reason to join the subject and description just to split again later. We only
want to change the description, so work with that directly.

This requires an extra "if" for the case that the description is empty, but I
think it's worth it.

Also, take this opportunity to add an extra blank line before the Co-authored-by
line, like we do when invoking it after the fact.

Author: stefanhaller

pkg/gui/controllers/helpers/commits_helper.go

@@ -215,9 +215,12 @@ func (self *CommitsHelper) addCoAuthor(suggestionFunc func(string) []*types.Sugg
 		Title:               self.c.Tr.AddCoAuthorPromptTitle,
 		FindSuggestionsFunc: suggestionFunc,
 		HandleConfirm: func(value string) error {
-			commitMessage := self.JoinCommitMessageAndDescription()
-			coAuthorString := commitMessage + "\nCo-authored-by: " + value
-			self.SetMessageAndDescriptionInView(coAuthorString)
+			commitDescription := self.getCommitDescription()
+			if len(commitDescription) != 0 {
+				commitDescription += "\n\n"
+			}
+			commitDescription += "Co-authored-by: " + value
+			self.setCommitDescription(commitDescription)
 			return nil
 		},
 	})