git-remote-testgit: import non-HEAD refs

git-remote-testgit: import non-HEAD refs

Upon receiving an "import" command, the testgit remote
helper would ignore the ref asked for by git and generate a
fast-export stream based on HEAD. Instead, we should
actually give git the ref it asked for.

This requires adding a new parameter to the export_repo
method in the remote-helpers python library, which may be
used by code outside of git.git. We use a default parameter
so that callers without the new parameter will get the same
behavior as before.

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