GIT_PREFIX was added in
7cf16a14f5c070f7b14cf28023769450133172ae so that
aliases can know the directory from which a !alias was called.
Knowing the prefix relative to the root is helpful in other programs
so export it to built-ins as well.
Helped-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>