Merge branch 'dd/diff-files-unmerged-fix' into maint

"git diff --relative" segfaulted and/or produced incorrect result
when there are unmerged paths.

* dd/diff-files-unmerged-fix:
  diff-lib: ignore paths that are outside $cwd if --relative asked

diff --cc diff-lib.c
Simple merge

diff --cc t/t4045-diff-relative.sh
Simple merge