Merge branch 'so/log-diff-merge'

"git log" learned "--diff-merges=<style>" option, with an
associated configuration variable log.diffMerges.

* so/log-diff-merge:
  doc/diff-options: document new --diff-merges features
  diff-merges: introduce log.diffMerges config variable
  diff-merges: adapt -m to enable default diff format
  diff-merges: refactor set_diff_merges()
  diff-merges: introduce --diff-merges=on

diff --cc builtin/log.c
Simple merge