credential-cache--daemon: disallow relative socket path

credential-cache--daemon: disallow relative socket path

Relative socket paths are dangerous since the user cannot generally
control when the daemon starts (initially, after a timeout, kill or
crash). Since the daemon creates but does not delete the socket
directory, this could lead to spurious directory creation relative
to the users cwd.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Jon Griffiths <jon_p_griffiths@yahoo.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>