built-in add -p: implement the "stash" and "reset" patch modes

built-in add -p: implement the "stash" and "reset" patch modes

The `git stash` and `git reset` commands support a `--patch` option, and
both simply hand off to `git add -p` to perform that work. Let's teach
the built-in version of that command to be able to perform that work, too.

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