contrib/subtree/Makefile: use GIT-VERSION-FILE
contrib/subtree/Makefile: use GIT-VERSION-FILE

GVF is already being used in most/all other makefiles in the project,
and has been for _quite_ a while. Hence, drop file-unique gitver and
replace with GIT_VERSION.

Reviewed-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: James Denholm <82f798d2ba97fb243e81a4141691a74fe6f8e42e@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git