Provide an environment variable GIT_PREFIX which contains the subdirectory
from which a !alias was called (i.e. 'git rev-parse --show-prefix') since
these cd to the to level directory before they are executed.
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>