show: tweak plain abort language for clarity

hgext/show.py

@@ -73,10 +73,8 b' def show(ui, repo, view=None, template=N'
 
     """
     if ui.plain() and not template:
-        raise error.Abort(_('"hg show" cannot be used in plain mode because '
-                            'output is not stable'),
-                          hint=_('unset HGPLAIN and invoke with -T/--template '
-                                 'to control output'))
+        hint = _('invoke with -T/--template to control output format')
+        raise error.Abort(_('must specify a template in plain mode'), hint=hint)
 
     views = showview._table
 

tests/test-show.t

@@ -55,8 +55,8 b' Unknown view prints error'
 HGPLAIN results in abort
 
   $ HGPLAIN=1 hg show bookmarks
-  abort: "hg show" cannot be used in plain mode because output is not stable
-  (unset HGPLAIN and invoke with -T/--template to control output)
+  abort: must specify a template in plain mode
+  (invoke with -T/--template to control output format)
   [255]
 
 But not if a template is specified