Merge branch 'ps/difftool-dir-diff-exit-code' into HEAD

"git difftool --dir-diff" learned to honor the "--trust-exit-code"
option; it used to always exit with 0 and signalled success.

* ps/difftool-dir-diff-exit-code:
  git-difftool--helper: honor `--trust-exit-code` with `--dir-diff`

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