Add 'verify-commit' to be used in a way similar to 'verify-tag' is
used.  Further work on verifying the mergetags might be needed.

* mg/verify-commit:
  t7510: test verify-commit
  t7510: exit for loop with test result
  verify-commit: scriptable commit signature verification
  gpg-interface: provide access to the payload
  gpg-interface: provide clear helper for struct signature_check