describe: use strbuf_add_unique_abbrev() for adding short hashes

Call strbuf_add_unique_abbrev() to add an abbreviated hash to a strbuf
instead of taking a detour through find_unique_abbrev() and its static
buffer.  This is shorter and a bit more efficient.

Patch generated by Coccinelle (and contrib/coccinelle/strbuf.cocci).

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/describe.c

@@ -380,7 +380,7 @@ static void describe_commit(struct object_id *oid, struct strbuf *dst)
 	if (!match_cnt) {
 		struct object_id *cmit_oid = &cmit->object.oid;
 		if (always) {
-			strbuf_addstr(dst, find_unique_abbrev(cmit_oid->hash, abbrev));
+			strbuf_add_unique_abbrev(dst, cmit_oid->hash, abbrev);
 			if (suffix)
 				strbuf_addstr(dst, suffix);
 			return;