Don't require a diff which accepts the -N option.

tests/test-extdiff

@@ -7,11 +7,7 b' hg init a'
 cd a
 echo a > a
 hg add
-diff -N /dev/null /dev/null 2> /dev/null
-if [ $? -ne 0 ]; then
-	opt="-p gdiff"
-fi
-hg extdiff -o -Nr $opt
+hg extdiff -o -r $opt
 
 echo "[extdiff]" >> $HGRCPATH
 echo "cmd.falabala=echo" >> $HGRCPATH

tests/test-extdiff.out

@@ -1,9 +1,7 b''
 adding a
 making snapshot of 0 files from rev 000000000000
 making snapshot of 1 files from working dir
-diff -Nr a.000000000000/a a/a
-0a1
-> a
+Only in a: a
 making snapshot of 0 files from rev 000000000000
 making snapshot of 1 files from working dir
 diffing a.000000000000 a