Merge branch 'jk/maint-null-in-trees'

We do not want a link to 0{40} object stored anywhere in our objects.

* jk/maint-null-in-trees:
  fsck: detect null sha1 in tree entries
  do not write null sha1s to on-disk index
  diff: do not use null sha1 as a sentinel value
diff --git a/diff-no-index.c b/diff-no-index.c
index b34b5aa..74da659 100644
--- a/diff-no-index.c
+++ b/diff-no-index.c
@@ -82,7 +82,7 @@
 	if (!name)
 		name = "/dev/null";
 	s = alloc_filespec(name);
-	fill_filespec(s, null_sha1, mode);
+	fill_filespec(s, null_sha1, 0, mode);
 	if (name == file_from_standard_input)
 		populate_from_stdin(s);
 	return s;