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