sequencer: convert some functions to struct object_id

Convert update_squash_messages and is_index_unchanged to struct
object_id.  These are callers of lookup_commit and
lookup_commit_reference, which we want to convert.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -482,13 +482,13 @@ static int do_recursive_merge(struct commit *base, struct commit *next,
 
 static int is_index_unchanged(void)
 {
-	unsigned char head_sha1[20];
+	struct object_id head_oid;
 	struct commit *head_commit;
 
-	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, head_sha1, NULL))
+	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, head_oid.hash, NULL))
 		return error(_("could not resolve HEAD commit\n"));
 
-	head_commit = lookup_commit(head_sha1);
+	head_commit = lookup_commit(head_oid.hash);
 
 	/*
 	 * If head_commit is NULL, check_commit, called from
@@ -835,13 +835,13 @@ static int update_squash_messages(enum todo_command command,
 		strbuf_splice(&buf, 0, eol - buf.buf, header.buf, header.len);
 		strbuf_release(&header);
 	} else {
-		unsigned char head[20];
+		struct object_id head;
 		struct commit *head_commit;
 		const char *head_message, *body;
 
-		if (get_sha1("HEAD", head))
+		if (get_oid("HEAD", &head))
 			return error(_("need a HEAD to fixup"));
-		if (!(head_commit = lookup_commit_reference(head)))
+		if (!(head_commit = lookup_commit_reference(head.hash)))
 			return error(_("could not read HEAD"));
 		if (!(head_message = get_commit_buffer(head_commit, NULL)))
 			return error(_("could not read HEAD's commit message"));