i18n: show localized messages for commands/extensions in hgweb help top (issue3383)

in hgweb help top page, help topics are localized, but abstracts of
commands and extensions are not, although these are already
translated.

it is because localized messages for them should be explicitly looked
up by original ones.

this patch looks localized messages up for each commands/extensions.

mercurial/hgweb/webcommands.py

@@ -806,7 +806,7 b' def graph(web, req, tmpl):'
 def _getdoc(e):
     doc = e[0].__doc__
     if doc:
-        doc = doc.split('\n')[0]
+        doc = _(doc).split('\n')[0]
     else:
         doc = _('(no help text available)')
     return doc