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

diff --cc Documentation/diff-options.txt
Simple merge

diff --cc diff.c
Simple merge

diff --cc diff.h
Simple merge