refs: adapt initial_transaction flag to be unsigned

The `initial_transaction` flag is tracked as a signed integer, but we
typically pass around flags via unsigned integers. Adapt the type
accordingly.

Suggested-by: Christian Couder <christian.couder@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -2325,7 +2325,7 @@ int refs_verify_refname_available(struct ref_store *refs,
 				  const char *refname,
 				  const struct string_list *extras,
 				  const struct string_list *skip,
-				  int initial_transaction,
+				  unsigned int initial_transaction,
 				  struct strbuf *err)
 {
 	const char *slash;