pkt: correctly advertise capabilitites

The correct way to advertise out capabilities is by appending them to
the first 'want' line, using SP as separator, instead of NUL as the
server does. Inconsistent documentation lead to the use of NUL in
libgit2.

Fix this so we can request much more efficient packs from the
remote which reduces the indexing time considerably.

src/pkt.c

@@ -296,7 +296,7 @@ static int buffer_want_with_caps(git_remote_head *head, git_transport_caps *caps
 	git_buf_grow(buf, git_buf_len(buf) + len);
 
 	git_oid_fmt(oid, &head->oid);
-	return git_buf_printf(buf, "%04xwant %s%c%s\n", len, oid, 0, capstr);
+	return git_buf_printf(buf, "%04xwant %s %s\n", len, oid, capstr);
 }
 
 /*