Merge branch 'jk/maint-null-in-trees' into maint-1.7.11

Merge branch 'jk/maint-null-in-trees' into maint-1.7.11

"git diff" had a confusion between taking data from a path in the
working tree and taking data from an object that happens to have
name 0{40} recorded in a tree.

* 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