Merge branch 'nd/grep-reflog'

Teach the commands from the "log" family the "--grep-reflog" option
to limit output by string that appears in the reflog entry when the
"--walk-reflogs" option is in effect.

* nd/grep-reflog:
  revision: make --grep search in notes too if shown
  log --grep-reflog: reject the option without -g
  revision: add --grep-reflog to filter commits by reflog messages
  grep: prepare for new header field filter