built-in add -i: implement the `patch` command

built-in add -i: implement the `patch` command

Well, it is not a full implementation yet. In the interest of making
this easy to review (and easy to keep bugs out), we still hand off to
the Perl script to do the actual work.

The `patch` functionality actually makes up for more than half of the
1,800+ lines of `git-add--interactive.perl`. It will be ported from Perl
to C incrementally, later.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>