dispatch: explicitly pass fancyopts optional arg as a keyword

I've been baffled by this a couple of times (mainly wondering if any
callers of fancyopts.fancyopts that don't use gnu=True exist), so
let's just specify this as a keyword argument to preserve sanity.

mercurial/dispatch.py

@@ -556,7 +556,7 b' def _parse(ui, args):'
         c.append((o[0], o[1], options[o[1]], o[3]))
 
     try:
-        args = fancyopts.fancyopts(args, c, cmdoptions, True)
+        args = fancyopts.fancyopts(args, c, cmdoptions, gnu=True)
     except fancyopts.getopt.GetoptError as inst:
         raise error.CommandError(cmd, inst)