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