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

diffcore.h

@@ -55,7 +55,7 @@ struct diff_filespec {
 extern struct diff_filespec *alloc_filespec(const char *);
 extern void free_filespec(struct diff_filespec *);
 extern void fill_filespec(struct diff_filespec *, const unsigned char *,
-			  unsigned short);
+			  int, unsigned short);
 
 extern int diff_populate_filespec(struct diff_filespec *, int);
 extern void diff_free_filespec_data(struct diff_filespec *);