sequencer: lib'ify read_populate_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 read_populate_todo(), sequencer_continue() 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 read_populate_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

@@ -748,7 +748,7 @@ static int parse_insn_buffer(char *buf, struct commit_list **todo_list,
 	return 0;
 }
 
-static void read_populate_todo(struct commit_list **todo_list,
+static int read_populate_todo(struct commit_list **todo_list,
 			struct replay_opts *opts)
 {
 	struct strbuf buf = STRBUF_INIT;
@@ -756,18 +756,21 @@ static void read_populate_todo(struct commit_list **todo_list,
 
 	fd = open(git_path_todo_file(), O_RDONLY);
 	if (fd < 0)
-		die_errno(_("Could not open %s"), git_path_todo_file());
+		return error_errno(_("Could not open %s"),
+				   git_path_todo_file());
 	if (strbuf_read(&buf, fd, 0) < 0) {
 		close(fd);
 		strbuf_release(&buf);
-		die(_("Could not read %s."), git_path_todo_file());
+		return error(_("Could not read %s."), git_path_todo_file());
 	}
 	close(fd);
 
 	res = parse_insn_buffer(buf.buf, todo_list, opts);
 	strbuf_release(&buf);
 	if (res)
-		die(_("Unusable instruction sheet: %s"), git_path_todo_file());
+		return error(_("Unusable instruction sheet: %s"),
+			git_path_todo_file());
+	return 0;
 }
 
 static int populate_opts_cb(const char *key, const char *value, void *data)
@@ -1019,7 +1022,8 @@ static int sequencer_continue(struct replay_opts *opts)
 	if (!file_exists(git_path_todo_file()))
 		return continue_single_pick();
 	read_populate_opts(&opts);
-	read_populate_todo(&todo_list, opts);
+	if (read_populate_todo(&todo_list, opts))
+		return -1;
 
 	/* Verify that the conflict has been resolved */
 	if (file_exists(git_path_cherry_pick_head()) ||