Merge branch 'jk/reflog-date'

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

diff --cc Documentation/rev-list-options.txt
Simple merge

diff --cc builtin/blame.c
Simple merge

diff --cc cache.h
Simple merge