rename_ref(): integrate lock_ref_sha1_basic() errors into ours

Now that lock_ref_sha1_basic() gives us back its error messages via a
strbuf, incorporate its error message into our error message rather
than emitting two separate error messages.
Signed-off-by: NMichael Haggerty <mhagger@alum.mit.edu>

refs.c

@@ -2907,9 +2907,8 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
 
 	lock = lock_ref_sha1_basic(newrefname, NULL, NULL, NULL, 0, NULL, &err);
 	if (!lock) {
-		error("%s", err.buf);
+		error("unable to rename '%s' to '%s': %s", oldrefname, newrefname, err.buf);
 		strbuf_release(&err);
-		error("unable to lock %s for update", newrefname);
 		goto rollback;
 	}
 	hashcpy(lock->old_sha1, orig_sha1);
@@ -2923,9 +2922,8 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
  rollback:
 	lock = lock_ref_sha1_basic(oldrefname, NULL, NULL, NULL, 0, NULL, &err);
 	if (!lock) {
-		error("%s", err.buf);
+		error("unable to lock %s for rollback: %s", oldrefname, err.buf);
 		strbuf_release(&err);
-		error("unable to lock %s for rollback", oldrefname);
 		goto rollbacklog;
 	}