send-email: lazily assign editor variable

send-email: lazily assign editor variable

b4479f0 (add -i, send-email, svn, p4, etc: use "git var GIT_EDITOR",
2009-10-30) introduced the use of "git var GIT_EDITOR" to obtain the
preferred editor program, instead of reading environment variables
themselves.

However, "git var GIT_EDITOR" run without a tty (think "cron job") would
give a fatal error "Terminal is dumb, but EDITOR unset".  This is not a
problem for add-i, svn, p4 and callers of git_editor() defined in
git-sh-setup, as all of these call it just before launching the editor.
At that point, we know the caller wants to edit.

But send-email ran this near the beginning of the program, even if it is
not going to use any editor (e.g. run without --compose).  Fix this by
calling the command only when we edit a file.

Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>