help: make help deprecated mention the extension

before this, you got an empty list of extensions, which was unhelpful

mercurial/help.py

@@ -474,7 +474,8 b' def help_(ui, name, unknowncmd=False, fu'
         doc = gettext(mod.__doc__).splitlines()[0]
 
         rst = listexts(_("'%s' is provided by the following "
-                              "extension:") % cmd, {ext: doc}, indent=4)
+                              "extension:") % cmd, {ext: doc}, indent=4,
+                       showdeprecated=True)
         rst.append('\n')
         rst.append(_('(use "hg help extensions" for information on enabling '
                    'extensions)\n'))

tests/test-extension.t

@@ -367,6 +367,15 b' hide outer repo'
 
   $ echo 'debugextension = !' >> $HGRCPATH
 
+Asking for help about a deprecated extension should do something useful:
+
+  $ hg help glog
+  'glog' is provided by the following extension:
+  
+      graphlog      command to view revision graphs from a shell (DEPRECATED)
+  
+  (use "hg help extensions" for information on enabling extensions)
+
 Extension module help vs command help:
 
   $ echo 'extdiff =' >> $HGRCPATH