Merge branch 'ds/format-patch-rfc-and-k'

The "-k" and "--rfc" options of "format-patch" will now error out
when used together, as one tells us not to add anything to the
title of the commit, and the other one tells us to add "RFC" in
addition to "PATCH".

* ds/format-patch-rfc-and-k:
  format-patch: ensure that --rfc and -k are mutually exclusive

diff --cc builtin/log.c
Simple merge

diff --cc t/t4014-format-patch.sh
Simple merge