Merge branch 'jk/diff-relative-cached-unmerged' into jch

'git diff --relative' running with '--cached' has been corrected to
avoid a segfault when encountering unmerged paths outside the
prefix.

* jk/diff-relative-cached-unmerged:
  diff-lib: add idx/tree sanity check to oneway_diff
  diff: ignore unmerged paths outside prefix with --relative --cached

diff --cc diff-lib.c
Simple merge