Merge branch 'rs/diff-caret-bang-with-parents'

"git diff rev^!" did not show combined diff to go to the rev from
its parents.

* rs/diff-caret-bang-with-parents:
  diff: support ^! for merges
  revisions.txt: unspecify order of resolved parts of ^!
  revision: use strtol_i() for exclude_parent

diff --cc Documentation/git-diff.txt
Simple merge

diff --cc revision.c
Simple merge