git-commit: partial commit of paths only removed from the index

git-commit: partial commit of paths only removed from the index

Because a partial commit is meant to be a way to ignore what are
staged in the index, "git rm --cached A && git commit A" should
just record what is in A on the filesystem.  The previous patch
made the command sequence to barf, saying that A has not been
added yet.  This fixes it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>