Merge branch 'jc/cat-file-batch-commands'

"git cat-file" learns "--batch-command" mode, which is a more
flexible interface than the existing "--batch" or "--batch-check"
modes, to allow different kinds of inquiries made.

* jc/cat-file-batch-commands:
  cat-file: add --batch-command mode
  cat-file: add remove_timestamp helper
  cat-file: introduce batch_mode enum to replace print_contents
  cat-file: rename cmdmode to transform_mode