git-stash clear: refuse to work with extra parameter for now

git-stash clear: refuse to work with extra parameter for now

Because it is so tempting to expect "git stash clear stash@{4}"
to remove the fourth element in the stash while leaving other
elements intact, we should not blindly throw away everything
upon seeing such a command.

This may change when we start using "git reflog delete" to
selectively nuke a single (or multiple, for that matter) stash
entries with such a command line.

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