fast-import: fix buffer overflow in dump_tags

When creating a new annotated tag, we sprintf the refname
into a static-sized buffer. If we have an absurdly long
tagname, like:

  git init repo &&
  cd repo &&
  git commit --allow-empty -m foo &&
  git tag -m message mytag &&
  git fast-export mytag |
  perl -lpe '/^tag/ and s/mytag/"a" x 8192/e' |
  git fast-import <input

we'll overflow the buffer. We can fix it by using a strbuf.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

fast-import.c

@@ -1730,14 +1730,16 @@ static void dump_tags(void)
 	static const char *msg = "fast-import";
 	struct tag *t;
 	struct ref_lock *lock;
-	char ref_name[PATH_MAX];
+	struct strbuf ref_name = STRBUF_INIT;
 
 	for (t = first_tag; t; t = t->next_tag) {
-		sprintf(ref_name, "tags/%s", t->name);
-		lock = lock_ref_sha1(ref_name, NULL);
+		strbuf_reset(&ref_name);
+		strbuf_addf(&ref_name, "tags/%s", t->name);
+		lock = lock_ref_sha1(ref_name.buf, NULL);
 		if (!lock || write_ref_sha1(lock, t->sha1, msg) < 0)
-			failure |= error("Unable to update %s", ref_name);
+			failure |= error("Unable to update %s", ref_name.buf);
 	}
+	strbuf_release(&ref_name);
 }
 
 static void dump_marks_helper(FILE *f,