xdiff: unignore changes in function context

xdiff: unignore changes in function context

Changes involving only blank lines are hidden with --ignore-blank-lines,
unless they appear in the context lines of other changes.  This is
handled by xdl_get_hunk() for context added by --inter-hunk-context, -u
and -U.

Function context for -W and --function-context added by xdl_emit_diff()
doesn't pay attention to such ignored changes; it relies fully on
xdl_get_hunk() and shows just the post-image of ignored changes
appearing in function context.  That's inconsistent and confusing.

Improve the result of using --ignore-blank-lines and --function-context
together by fully showing ignored changes if they happen to fall within
function context.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>