sequencer: replace write_cherry_pick_head with update_ref

Now update_ref (via write_pseudoref) does almost exactly what
write_cherry_pick_head did, so we can remove write_cherry_pick_head
and just use update_ref.

Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -158,23 +158,6 @@ static void free_message(struct commit *commit, struct commit_message *msg)
 	unuse_commit_buffer(commit, msg->message);
 }
 
-static void write_cherry_pick_head(struct commit *commit, const char *pseudoref)
-{
-	const char *filename;
-	int fd;
-	struct strbuf buf = STRBUF_INIT;
-
-	strbuf_addf(&buf, "%s\n", sha1_to_hex(commit->object.sha1));
-
-	filename = git_path("%s", pseudoref);
-	fd = open(filename, O_WRONLY | O_CREAT, 0666);
-	if (fd < 0)
-		die_errno(_("Could not open '%s' for writing"), filename);
-	if (write_in_full(fd, buf.buf, buf.len) != buf.len || close(fd))
-		die_errno(_("Could not write to '%s'"), filename);
-	strbuf_release(&buf);
-}
-
 static void print_advice(int show_hint, struct replay_opts *opts)
 {
 	char *msg = getenv("GIT_CHERRY_PICK_HELP");
@@ -607,9 +590,11 @@ static int do_pick_commit(struct commit *commit, struct replay_opts *opts)
 	 * write it at all.
 	 */
 	if (opts->action == REPLAY_PICK && !opts->no_commit && (res == 0 || res == 1))
-		write_cherry_pick_head(commit, "CHERRY_PICK_HEAD");
+		update_ref(NULL, "CHERRY_PICK_HEAD", commit->object.sha1, NULL,
+			   REF_NODEREF, UPDATE_REFS_DIE_ON_ERR);
 	if (opts->action == REPLAY_REVERT && ((opts->no_commit && res == 0) || res == 1))
-		write_cherry_pick_head(commit, "REVERT_HEAD");
+		update_ref(NULL, "REVERT_HEAD", commit->object.sha1, NULL,
+			   REF_NODEREF, UPDATE_REFS_DIE_ON_ERR);
 
 	if (res) {
 		error(opts->action == REPLAY_REVERT