Merge branch 'jk/strbuf-addftime-seconds-since-epoch'

The "--date=format:<strftime>" gained a workaround for the lack of
system support for a non-local timezone to handle "%s" placeholder.

* jk/strbuf-addftime-seconds-since-epoch:
  strbuf_addftime(): handle "%s" manually

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