apply --recount: allow "no-op hunks"

apply --recount: allow "no-op hunks"

When editing patches e.g. in `git add -e`, it is quite common that a
hunk ends up having no -/+ lines, i.e. it is now supposed to do nothing.

This use case was broken by ad6e8ed37bc1 (apply: reject a hunk that does
not do anything, 2015-06-01) with the good intention of catching a very
real, different issue in hand-edited patches.

So let's use the `--recount` option as the tell-tale whether the user
would actually be okay with no-op hunks.

Add a test case to make sure that this use case does not regress again.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>