Merge branch 'jk/diff-color-moved-fix' into maint

Merge branch 'jk/diff-color-moved-fix' into maint

The experimental "color moved lines differently in diff output"
feature was buggy around "ignore whitespace changes" edges, whihch
has been corrected.

* jk/diff-color-moved-fix:
  diff: handle NULs in get_string_hash()
  diff: fix whitespace-skipping with --color-moved
  t4015: test the output of "diff --color-moved -b"
  t4015: check "negative" case for "-w --color-moved"
  t4015: refactor --color-moved whitespace test