built-in add -i: do not try to `patch`/`diff` an empty list of files

built-in add -i: do not try to `patch`/`diff` an empty list of files

When the user does not select any files to `patch` or `diff`, there is
no need to call `run_add_p()` on them.

Even worse: we _have_ to avoid calling `parse_pathspec()` with an empty
list because that would trigger this error:

	BUG: pathspec.c:557: PATHSPEC_PREFER_CWD requires arguments

So let's avoid doing any work on a list of files that is empty anyway.

This fixes https://github.com/git-for-windows/git/issues/2466.

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