ref_transaction_update(): remove "have_old" parameter

Instead, verify the reference's old value if and only if old_sha1 is
non-NULL.

ref_transaction_delete() will get the same treatment in a moment.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -1767,7 +1767,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	    ref_transaction_update(transaction, "HEAD", sha1,
 				   current_head
 				   ? current_head->object.sha1 : NULL,
-				   0, !!current_head, sb.buf, &err) ||
+				   0, sb.buf, &err) ||
 	    ref_transaction_commit(transaction, &err)) {
 		rollback_index_files();
 		die("%s", err.buf);