Merge branch 'jc/diff-ws-error-highlight'

Allow whitespace breakages in deleted and context lines to be also painted in the output. * jc/diff-ws-error-highlight: diff.c: --ws-error-highlight=<kind> option diff.c: add emit_del_line() and emit_context_line() t4015: separate common setup and per-test expectation t4015: modernise style