export: explicitly honor all diffopts

This is slightly more controversial than diff, but we hope that HGPLAIN=1
covers all the format-breaking ones.

A possible alternative here that breaks BC is to honor all opts except the
whitespace ones.

mercurial/commands.py

@@ -3169,7 +3169,7 b' def export(ui, repo, *changesets, **opts'
         ui.note(_('exporting patch:\n'))
     cmdutil.export(repo, revs, template=opts.get('output'),
                  switch_parent=opts.get('switch_parent'),
-                 opts=patch.diffopts(ui, opts))
+                 opts=patch.diffallopts(ui, opts))
 
 @command('files',
     [('r', 'rev', '', _('search the repository as it is in REV'), _('REV')),