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