add -i, send-email, svn, p4, etc: use "git var GIT_EDITOR"

Use the new "git var GIT_EDITOR" feature to decide what editor to
use, instead of duplicating its logic elsewhere.  This should make
the behavior of commands in edge cases (e.g., editor names with
spaces) a little more consistent.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>