Teach git-add and git-commit to handle filenames starting with '-'.

Teach git-add and git-commit to handle filenames starting with '-'.

Recent '--' fixes to "git diff" by Linus made it possible to specify
filenames that start with '-'.  But in order to do that, you need to
be able to add and commit such file to begin with.

Teach git-add and git-commit to honor the same '--' convention.

Signed-off-by: Junio C Hamano <junkio@cox.net>