pull,rebase: handle GIT_WORK_TREE better

pull,rebase: handle GIT_WORK_TREE better

You can't currently run git-pull or git-rebase from outside
of the work tree, even with GIT_WORK_TREE set, due to an
overeager require_work_tree function. Commit e2eb527
documents this problem and provides the infrastructure for a
fix, but left it to later commits to audit and update
individual scripts.

Changing these scripts to use require_work_tree_exists is
easy to verify. We immediately call cd_to_toplevel, anyway.
Therefore no matter which function we use, the state
afterwards is one of:

  1. We have a work tree, and we are at the top level.

  2. We don't have a work tree, and we have died.

The only catch is that we must also make sure no code that
ran before the cd_to_toplevel assumed that we were already
in the working tree.

In this case, we will only have included shell libraries and
called set_reflog_action, neither of which care about the
current working directory at all.

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