Merge branch 'en/remerge-diff'

"git log --remerge-diff" shows the difference from mechanical merge
result and the result that is actually recorded in a merge commit.

* en/remerge-diff:
  diff-merges: avoid history simplifications when diffing merges
  merge-ort: mark conflict/warning messages from inner merges as omittable
  show, log: include conflict/warning messages in --remerge-diff headers
  diff: add ability to insert additional headers for paths
  merge-ort: format messages slightly different for use in headers
  merge-ort: mark a few more conflict messages as omittable
  merge-ort: capture and print ll-merge warnings in our preferred fashion
  ll-merge: make callers responsible for showing warnings
  log: clean unneeded objects during `log --remerge-diff`
  show, log: provide a --remerge-diff capability

diff --cc apply.c
Simple merge

diff --cc builtin/checkout.c
Simple merge

diff --cc builtin/log.c
Simple merge

diff --cc diff.c
Simple merge

diff --cc merge-ort.c
Simple merge

diff --cc revision.h
Simple merge

diff --cc t/t6406-merge-attr.sh
Simple merge