[PATCH] git-write-tree doesn't check alternate directories

git-write-tree failed when referenced objects only exist in the
GIT_ALTERNATE_OBJECT_DIRECTORIES path.
Signed-off-by: NJan Harkes <jaharkes@cs.cmu.edu>
Acked-by: NJunio C Hamano <junkio@cox.net>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

write-tree.c

@@ -7,14 +7,13 @@
 
 static int check_valid_sha1(unsigned char *sha1)
 {
-	char *filename = sha1_file_name(sha1);
 	int ret;
 
 	/* If we were anal, we'd check that the sha1 of the contents actually matches */
-	ret = access(filename, R_OK);
-	if (ret)
-		perror(filename);
-	return ret;
+	ret = has_sha1_file(sha1);
+	if (ret == 0)
+		perror(sha1_file_name(sha1));
+	return ret ? 0 : -1;
 }
 
 static int write_tree(struct cache_entry **cachep, int maxentries, const char *base, int baselen, unsigned char *returnsha1)