When using the '-w $cvsdir' option to cvsexportcommit, it will chdir into
$cvsdir before executing several other git commands. If $GIT_DIR is set to
a relative path (e.g. '.'), the git commands executed by cvsexportcommit
will naturally fail.

Therefore, ensure that $GIT_DIR is absolute before the chdir to $cvsdir.
Signed-off-by: NJohan Herland <johan@herland.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>