Merge branch 'jk/reflog-date' into maint

The reflog output format is documented better, and a new format --date=unix to report the seconds-since-epoch (without timezone) has been added. * jk/reflog-date: date: clarify --date=raw description date: add "unix" format date: document and test "raw-local" mode doc/pretty-formats: explain shortening of %gd doc/pretty-formats: describe index/time formats for %gd doc/rev-list-options: explain "-g" output formats doc/rev-list-options: clarify "commit@{Nth}" for "-g" option