Refactor git-recent-branches.

* Remove use of perl.
* Make "max age" and "ignore_re" variables, which will support user
  settings in the future.
Add --since and --ignore to git-recent-branches.
git-recent-branches: validate --since value
git-recent-branches: support empty value for --since
git-recent-branches: read defaults from git-config

Also clean up the help message.