reset: unbreak hard resets with GIT_WORK_TREE

reset: unbreak hard resets with GIT_WORK_TREE

Commit 952dfc6 tried to tighten the safety valves for doing
a "reset --hard" in a bare repository or outside the work
tree, but accidentally broke the case for GIT_WORK_TREE.
This patch unbreaks it.

Most git commands which need a work tree simply use
NEED_WORK_TREE in git.c to die before they get to their
cmd_* function. Reset, however, only needs a work tree in
some cases, and so must handle the work tree itself. The
error that 952dfc6 made was to simply forbid certain
operations if the work tree was not set up; instead, we need
to do the same thing that NEED_WORK_TREE does, which is to
call setup_work_tree(). We no longer have to worry about dying
in the non-worktree case, as setup_work_tree handles that
for us.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>