Merge branch 'rs/external-diff-with-exit-code'

The "--exit-code" option of "git diff" command learned to work with
the "--ext-diff" option.

* rs/external-diff-with-exit-code:
  diff: fix --exit-code with external diff
  diff: report unmerged paths as changes in run_diff_cmd()

diff --cc diff.c
Simple merge

diff --cc t/t4020-diff-external.sh
Simple merge

diff --cc t/t4046-diff-unmerged.sh
Simple merge