t5801: make remote-testgit GIT_DIR setup more robust

Our tests use a fake helper that just imports from an existing Git
repository. We're fed the path to that repo on the command line, and
derive the GIT_DIR by tacking on "/.git".

This is wrong if the path is a bare repository, but that's OK since this
is just a limited test. But it's also wrong if the transport code feeds
us the actual .git directory itself (i.e., we expect "/path/to/repo" but
it gives us "/path/to/repo/.git"). None of the current tests do that,
but let's future-proof ourselves against adding a test that does.

We can instead ask "rev-parse" to set our GIT_DIR. Note that we have to
first unset other git variables from our environment. Coming into this
script, we'll have GIT_DIR set to the fetching repository, and we need
to "switch" to the remote one.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>