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>

git-am.sh

@@ -446,6 +446,8 @@ done
 # If the dotest directory exists, but we have finished applying all the
 # patches in them, clear it out.
 if test -d "$dotest" &&
+   test -f "$dotest/last" &&
+   test -f "$dotest/next" &&
    last=$(cat "$dotest/last") &&
    next=$(cat "$dotest/next") &&
    test $# != 0 &&
@@ -454,7 +456,7 @@ then
    rm -fr "$dotest"
 fi
 
-if test -d "$dotest"
+if test -d "$dotest" && test -f "$dotest/last" && test -f "$dotest/next"
 then
 	case "$#,$skip$resolved$abort" in
 	0,*t*)