add-patch: add support for in-memory index patching

add-patch: add support for in-memory index patching

With `run_add_p()` callers have the ability to apply changes from a
specific revision to a repository's index. This infra supports several
different modes, like for example applying changes to the index,
working tree or both.

One feature that is missing though is the ability to apply changes to an
in-memory index different from the repository's index. Add a new
function `run_add_p_index()` to plug this gap.

This new function will be used in a subsequent commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>