legacy stash -p: respect the add.interactive.usebuiltin setting

legacy stash -p: respect the add.interactive.usebuiltin setting

As `git add` traditionally did not expose the `--patch=<mode>` modes via
command-line options, the scripted version of `git stash` had to call
`git add--interactive` directly.

But this prevents the built-in `add -p` from kicking in, as
`add--interactive` is the scripted version (which does not have a
"fall-back" to the built-in version).

So let's introduce support for internal switch for `git add` that the
scripted `git stash` can use to call the appropriate backend (scripted
or built-in, depending on `add.interactive.useBuiltin`).

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