git: update to v2.10.0

Upstream continues to replace unsigned char *sha1 with struct
object_id old_oid. This makes the required changes.

The git lib has its own main function now. Rename our main function
to cmd_main, it is called from main then.

cgit.c

@@ -1026,7 +1026,7 @@ static int calc_ttl(void)
 	return ctx.cfg.cache_repo_ttl;
 }
 
-int main(int argc, const char **argv)
+int cmd_main(int argc, const char **argv)
 {
 	const char *path;
 	int err, ttl;