sequencer: lib'ify save_todo()

Instead of dying there, let the caller high up in the callchain notice
the error and handle it (by dying, still).

The only caller of save_todo(), pick_commits() can already return
errors, so its caller must be already prepared to handle error
returns, and with this step, we make it notice an error return from
this function.

So this is a safe conversion to make save_todo() callable
from new callers that want it not to die, without changing the
external behaviour of anything existing.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -943,24 +943,31 @@ fail:
 	return -1;
 }
 
-static void save_todo(struct commit_list *todo_list, struct replay_opts *opts)
+static int save_todo(struct commit_list *todo_list, struct replay_opts *opts)
 {
 	static struct lock_file todo_lock;
 	struct strbuf buf = STRBUF_INIT;
 	int fd;
 
-	fd = hold_lock_file_for_update(&todo_lock, git_path_todo_file(), LOCK_DIE_ON_ERROR);
-	if (format_todo(&buf, todo_list, opts) < 0)
-		die(_("Could not format %s."), git_path_todo_file());
+	fd = hold_lock_file_for_update(&todo_lock, git_path_todo_file(), 0);
+	if (fd < 0)
+		return error_errno(_("Could not lock '%s'"),
+				   git_path_todo_file());
+	if (format_todo(&buf, todo_list, opts) < 0) {
+		strbuf_release(&buf);
+		return error(_("Could not format %s."), git_path_todo_file());
+	}
 	if (write_in_full(fd, buf.buf, buf.len) < 0) {
 		strbuf_release(&buf);
-		die_errno(_("Could not write to %s"), git_path_todo_file());
+		return error_errno(_("Could not write to %s"),
+				   git_path_todo_file());
 	}
 	if (commit_lock_file(&todo_lock) < 0) {
 		strbuf_release(&buf);
-		die(_("Error wrapping up %s."), git_path_todo_file());
+		return error(_("Error wrapping up %s."), git_path_todo_file());
 	}
 	strbuf_release(&buf);
+	return 0;
 }
 
 static void save_opts(struct replay_opts *opts)
@@ -1009,7 +1016,8 @@ static int pick_commits(struct commit_list *todo_list, struct replay_opts *opts)
 		return -1;
 
 	for (cur = todo_list; cur; cur = cur->next) {
-		save_todo(cur, opts);
+		if (save_todo(cur, opts))
+			return -1;
 		res = do_pick_commit(cur->item, opts);
 		if (res)
 			return res;