am: tighten a conditional that checks for $dotest

In preparation for a later patch that creates $dotest/autostash in
git-rebase.sh before anything else happens, don't assume that the
presence of a $dotest directory implies the existence of the
$dotest/next and $dotest/last files.  Look for them explicitly.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>