git-jump: move valid-mode check earlier

git-jump: move valid-mode check earlier

We check if the "mode" argument supplied by the user is valid by seeing
if we have a mode_$mode function defined. But we don't do that until
after creating the tempfile. This is wasteful (we create a tempfile but
never use it), and makes it harder to add new options (the recent stdout
option exits before creating the tempfile, so it misses the check and
"git jump --stdout foo" will produce "git-jump: 92: mode_foo: not found"
rather than the regular usage message).

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