alias: show one-line hint for command provided by disabled extension

It will be a hint of Abort exception.  "hg help <alias>" provides the detailed
version as before.

mercurial/dispatch.py

@@ -447,7 +447,9 b' class cmdalias(object):'
             if self.unknowncmd:
                 try:
                     # check if the command is in a disabled extension
-                    commands.help_(ui, self.cmdname, unknowncmd=True)
+                    cmd, ext = extensions.disabledcmd(ui, self.cmdname)[:2]
+                    ui.warn(_("'%s' is provided by '%s' extension\n")
+                            % (cmd, ext))
                 except error.UnknownCommand:
                     pass
             return -1

tests/test-alias.t

@@ -90,11 +90,7 b' disabled'
 
   $ hg disabled
   alias 'disabled' resolves to unknown command 'email'
-  'email' is provided by the following extension:
-  
-      patchbomb     command to send changesets as (a series of) patch emails
-  
-  (use "hg help extensions" for information on enabling extensions)
+  'email' is provided by 'patchbomb' extension
   [255]
   $ hg help disabled
   alias 'disabled' resolves to unknown command 'email'