Make git commit --interactive feel more like git add --interactive by
allowing the user to restrict the list of files they have to deal with.

A test in t7501 used to ensure that this is not allowed; no need for that
anymore.
Signed-off-by: NConrad Irwin <conrad.irwin@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>