strbuf_readlink: use ssize_t

The return type of readlink() is ssize_t, not int. This
probably doesn't matter in practice, as it would require a
2GB symlink destination, but it doesn't hurt to be careful.

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

strbuf.c

@@ -469,7 +469,7 @@ int strbuf_readlink(struct strbuf *sb, const char *path, size_t hint)
 		hint = 32;
 
 	while (hint < STRBUF_MAXLINK) {
-		int len;
+		ssize_t len;
 
 		strbuf_grow(sb, hint);
 		len = readlink(path, sb->buf, hint);