isequal: add --skip for both left and right file

isequal: New option --skip as a shortcut for --skipleft and --skipright
at once.

Use that for the .list-ref files, so they now contain the first line of
the output, too, which is ignored.

test/asm/listing/002-macro-param-listing.list-ref

@@ -1,3 +1,4 @@
+ca65 Vx.xx - Git XXXXXXXXX
 Main file   : 002-macro-param-listing.s
 Current file: 002-macro-param-listing.s