Merge branch 'mk/diff'

* mk/diff:
  Diff between two blobs should take mode changes into account now.
  use mode of the tree in git-diff, if <tree>:<file> syntax is used
  store mode in rev_list, if <tree>:<filename> syntax is used
  add add_object_array_with_mode
  add get_sha1_with_mode
  Add S_IFINVALID mode