Merge branch 'vd/cat-file-objectmode-update'

"git cat-file --batch" learns to understand %(objectmode) atom to
allow the caller to tell missing objects (due to repository
corruption) and submodules (whose commit objects are OK to be
missing) apart.

* vd/cat-file-objectmode-update:
  cat-file.c: add batch handling for submodules
  cat-file: add %(objectmode) atom
  t1006: update 'run_tests' to test generic object specifiers