git: update to v2.16.0

Update to git version v2.16.0:

* refs: convert resolve_ref_unsafe to struct object_id
  (49e61479be913f67e66bb3fdf8de9475c41b58bd)
* diff: remove DIFF_OPT_SET macro
  (23dcf77f48feb49c54bad09210f093a799816334)
* log: add option to choose which refs to decorate
  (65516f586b69307f977cd67cc45513a296cabc25)
* diff: convert flags to be stored in bitfields
  (02f2f56bc377c287c411947d0e1482aac888f8db)

Signed-off-by: Christian Hesse <mail@eworm.de>

cgit.c

@@ -478,7 +478,7 @@ static char *guess_defbranch(void)
 	const char *ref, *refname;
 	struct object_id oid;
 
-	ref = resolve_ref_unsafe("HEAD", 0, oid.hash, NULL);
+	ref = resolve_ref_unsafe("HEAD", 0, &oid, NULL);
 	if (!ref || !skip_prefix(ref, "refs/heads/", &refname))
 		return "master";
 	return xstrdup(refname);