sequencer: rectify empty hint in call of require_clean_work_tree()

The canonical way to represent "no error hint" is making it NULL, which
shortcuts the error() call altogether. This fixes the output by removing
the line which said just "error:", which would appear when the worktree
is dirtied while editing the initial rebase todo file. This was
introduced by 97e1873 (rebase -i: rewrite complete_action() in C,
2018-08-28), which did a somewhat inaccurate conversion from shell.

To avoid that such bugs re-appear, test for the condition in
require_clean_work_tree().

Signed-off-by: Oswald Buddenhagen <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ossilator

sequencer.c

@@ -6172,7 +6172,7 @@ int complete_action(struct repository *r, struct replay_opts *opts, unsigned fla
 	if (checkout_onto(r, opts, onto_name, &oid, orig_head))
 		goto cleanup;
 
-	if (require_clean_work_tree(r, "rebase", "", 1, 1))
+	if (require_clean_work_tree(r, "rebase", NULL, 1, 1))
 		goto cleanup;
 
 	todo_list_write_total_nr(&new_todo);

wt-status.c

@@ -2651,8 +2651,12 @@ int require_clean_work_tree(struct repository *r,
 	}
 
 	if (err) {
-		if (hint)
+		if (hint) {
+			if (!*hint)
+				BUG("empty hint passed to require_clean_work_tree();"
+				    " use NULL instead");
 			error("%s", hint);
+		}
 		if (!gently)
 			exit(128);
 	}