remote-helpers: export GIT_DIR variable to helpers

remote-helpers: export GIT_DIR variable to helpers

The gitdir capability is recognized by git and can be used to tell
the helper where the .git directory is. But it is not mentioned in
the documentation and considered worse than if gitdir was passed
via GIT_DIR environment variable.

Remove support for the gitdir capability and export GIT_DIR instead.
Teach testgit to use env instead of the now-removed gitdir command.

[sr: fixed up documentation]

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>