Fix 'git cvsexportcommit -w $cvsdir ...' when used with relative $GIT_DIR

Fix 'git cvsexportcommit -w $cvsdir ...' when used with relative $GIT_DIR

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: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>