Merge branch 'rs/diff-exit-code-fix'

In a few corner cases "git diff --exit-code" failed to report
"changes" (e.g., renamed without any content change), which has
been corrected.

* rs/diff-exit-code-fix:
  diff: report dirty submodules as changes in builtin_diff()
  diff: report copies and renames as changes in run_diff_cmd()

diff --cc diff.c
Simple merge