fix parsing of @{-1}@{u} combination

Previously interpret_branch_name would see @{-1} and stop
parsing, leaving the @{u} as cruft that provoked an error.
Instead, we should recurse if there is more to parse.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_name.c

@@ -881,8 +881,28 @@ int interpret_branch_name(const char *name, struct strbuf *buf)
 
 	if (!len)
 		return len; /* syntax Ok, not enough switches */
-	if (0 < len)
-		return len; /* consumed from the front */
+	if (0 < len && len == namelen)
+		return len; /* consumed all */
+	else if (0 < len) {
+		/* we have extra data, which might need further processing */
+		struct strbuf tmp = STRBUF_INIT;
+		int used = buf->len;
+		int ret;
+
+		strbuf_add(buf, name + len, namelen - len);
+		ret = interpret_branch_name(buf->buf, &tmp);
+		/* that data was not interpreted, remove our cruft */
+		if (ret < 0) {
+			strbuf_setlen(buf, used);
+			return len;
+		}
+		strbuf_reset(buf);
+		strbuf_addbuf(buf, &tmp);
+		strbuf_release(&tmp);
+		/* tweak for size of {-N} versus expanded ref name */
+		return ret - used + len;
+	}
+
 	cp = strchr(name, '@');
 	if (!cp)
 		return -1;

t/t1508-at-combinations.sh

@@ -43,8 +43,8 @@ check "@{-1}" old-two
 check "@{-1}@{1}" old-one
 check "@{u}" upstream-two
 check "@{u}@{1}" upstream-one
-fail check "@{-1}@{u}" master-two
-fail check "@{-1}@{u}@{1}" master-one
+check "@{-1}@{u}" master-two
+check "@{-1}@{u}@{1}" master-one
 fail nonsense "@{u}@{-1}"
 nonsense "@{1}@{u}"