read_packed_refs(): report unexpected fopen() failures

The old code ignored any errors encountered when trying to fopen the
"packed-refs" file, treating all such failures as if the file didn't
exist. But it could be that there is some other error opening the
file (e.g., permissions problems), and we don't want to silently
ignore such problems. So report any failures that are not due to
ENOENT.

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

refs/files-backend.c

@@ -251,8 +251,18 @@ static struct packed_ref_cache *read_packed_refs(const char *packed_refs_file)
 	packed_refs->cache->root->flag &= ~REF_INCOMPLETE;
 
 	f = fopen(packed_refs_file, "r");
-	if (!f)
-		return packed_refs;
+	if (!f) {
+		if (errno == ENOENT) {
+			/*
+			 * This is OK; it just means that no
+			 * "packed-refs" file has been written yet,
+			 * which is equivalent to it being empty.
+			 */
+			return packed_refs;
+		} else {
+			die_errno("couldn't read %s", packed_refs_file);
+		}
+	}
 
 	stat_validity_update(&packed_refs->validity, fileno(f));