Merge branch 'cc/git-packet-pm'

Parts of a test to drive the long-running content filter interface
has been split into its own module, hopefully to eventually become
reusable.

* cc/git-packet-pm:
  Git/Packet.pm: extract parts of t0021/rot13-filter.pl for reuse
  t0021/rot13-filter: add capability functions
  t0021/rot13-filter: refactor checking final lf
  t0021/rot13-filter: add packet_initialize()
  t0021/rot13-filter: improve error message
  t0021/rot13-filter: improve 'if .. elsif .. else' style
  t0021/rot13-filter: refactor packet reading functions
  t0021/rot13-filter: fix list comparison