Merge branch 'pw/add-p-recount'

When user edits the patch in "git add -p" and the user's editor is
set to strip trailing whitespaces indiscriminately, an empty line
that is unchanged in the patch would become completely empty
(instead of a line with a sole SP on it).  The code introduced in
Git 2.17 timeframe failed to parse such a patch, but now it learned
to notice the situation and cope with it.

* pw/add-p-recount:
  add -p: fix counting empty context lines in edited patches

diff --cc git-add--interactive.perl
Simple merge

diff --cc t/t3701-add-interactive.sh
Simple merge