Merge branch 'so/log-diff-merge'

Merge branch 'so/log-diff-merge'

"git log" learned a new "--diff-merges=<how>" option.

* so/log-diff-merge: (32 commits)
  t4013: add tests for --diff-merges=first-parent
  doc/git-show: include --diff-merges description
  doc/rev-list-options: document --first-parent changes merges format
  doc/diff-generate-patch: mention new --diff-merges option
  doc/git-log: describe new --diff-merges options
  diff-merges: add '--diff-merges=1' as synonym for 'first-parent'
  diff-merges: add old mnemonic counterparts to --diff-merges
  diff-merges: let new options enable diff without -p
  diff-merges: do not imply -p for new options
  diff-merges: implement new values for --diff-merges
  diff-merges: make -m/-c/--cc explicitly mutually exclusive
  diff-merges: refactor opt settings into separate functions
  diff-merges: get rid of now empty diff_merges_init_revs()
  diff-merges: group diff-merge flags next to each other inside 'rev_info'
  diff-merges: split 'ignore_merges' field
  diff-merges: fix -m to properly override -c/--cc
  t4013: add tests for -m failing to override -c/--cc
  t4013: support test_expect_failure through ':failure' magic
  diff-merges: revise revs->diff flag handling
  diff-merges: handle imply -p on -c/--cc logic for log.c
  ...