refs: allow skipping the reference-transaction hook

The reference-transaction hook is executing whenever we prepare, commit
or abort a reference transaction. While this is mostly intentional, in
case of the files backend we're leaking the implementation detail that
the store is in fact a composite store with one loose and one packed
backend to the caller. So while we want to execute the hook for all
logical updates, executing it for such implementation details is
unexpected.

Prepare for a fix by adding a new flag which allows to skip execution of
the hook.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -2084,6 +2084,9 @@ static int run_transaction_hook(struct ref_transaction *transaction,
 	const char *hook;
 	int ret = 0, i;
 
+	if (transaction->flags & REF_TRANSACTION_SKIP_HOOK)
+		return 0;
+
 	hook = find_hook("reference-transaction");
 	if (!hook)
 		return ret;

refs.h

@@ -568,6 +568,11 @@ enum action_on_err {
 	UPDATE_REFS_QUIET_ON_ERR
 };
 
+/*
+ * Skip executing the reference-transaction hook.
+ */
+#define REF_TRANSACTION_SKIP_HOOK (1 << 0)
+
 /*
  * Begin a reference transaction.  The reference transaction must
  * be freed by calling ref_transaction_free().