Merge branch 'ks/ref-filter-signature'

The "git for-each-ref" family of commands learned placeholders
related to GPG signature verification.

* ks/ref-filter-signature:
  ref-filter: add new "signature" atom
  t/lib-gpg: introduce new prereq GPG2

diff --cc ref-filter.c
Simple merge

diff --cc t/lib-gpg.sh
Simple merge