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>