built-in add -i: wire up the new C code for the `patch` command

built-in add -i: wire up the new C code for the `patch` command

The code in `git-add--interactive.perl` that takes care of the `patch`
command can look quite intimidating. There are so many modes in which it
can be called, for example.

But for the `patch` command in `git add -i`, only one mode is relevant:
the `stage` mode. And we just implemented the beginnings of that mode in
C so far. So let's use it when `add.interactive.useBuiltin=true`.

Now, while the code in `add-patch.c` is far from reaching feature parity
with the code in `git-add--interactive.perl` (color is not implemented,
the diff algorithm cannot be configured, the colored diff cannot be
post-processed via `interactive.diffFilter`, many commands are
unimplemented yet, etc), hooking it all up with the part of `git add -i`
that is already converted to C makes it easier to test and develop it.

Note: at this stage, both the `add.interactive.useBuiltin` config
setting is still safely opt-in, and will probably be fore quite some
time, to allow for thorough testing "in the wild" without adversely
affecting existing users.

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