Merge branch 'en/remerge-diff-fixes'

Fix a few "git log --remerge-diff" bugs.

* en/remerge-diff-fixes:
  diff: fix filtering of merge commits under --remerge-diff
  diff: fix filtering of additional headers under --remerge-diff
  diff: have submodule_format logic avoid additional diff headers

diff --cc diff.c
Simple merge