diff --git a/tests/test-bisect b/tests/test-bisect
--- a/tests/test-bisect
+++ b/tests/test-bisect
@@ -17,7 +17,7 @@ while test $count -lt 32 ; do
     test $count -eq 0 && hg add
     hg ci -m "msg $count" -d "$count 0"
     echo % committed changeset $count
-    count=$(( $count + 1 ))
+    count=`expr $count + 1`
 done
 
 echo % log
diff --git a/tests/test-extdiff b/tests/test-extdiff
--- a/tests/test-extdiff
+++ b/tests/test-extdiff
@@ -8,7 +8,11 @@ hg init a
 cd a
 echo a > a
 hg add
-hg extdiff -o -Nr
+diff -N /dev/null /dev/null 2> /dev/null
+if [ $? -ne 0 ]; then
+	opt="-p gdiff"
+fi
+hg extdiff -o -Nr $opt
 
 echo "[extdiff]" >> $HGTMP/.hgrc
 echo "cmd.falabala=echo" >> $HGTMP/.hgrc