httppeer: consolidate _requestbuilder assignments and document

I collapsed multiple assignments because they don't appear to
be necessary. We don't invoke the requestbuilder in anything
called during __init__. So there's no reason to not define it
earlier AFAICT.

This seemingly useless attribute is actually an extension point.
Document it as such.

(A previous version of this patch removed the attribute because
it appeared to just be an alias.)

Differential Revision: https://phab.mercurial-scm.org/D2723

mercurial/httppeer.py

@@ -138,7 +138,9 b' class httppeer(wireproto.wirepeer):'
         self._path = path
         self._caps = None
         self._urlopener = None
-        self._requestbuilder = None
+        # This is an its own attribute to facilitate extensions overriding
+        # the default type.
+        self._requestbuilder = urlreq.request
         u = util.url(path)
         if u.query or u.fragment:
             raise error.Abort(_('unsupported URL component: "%s"') %
@@ -151,7 +153,6 b' class httppeer(wireproto.wirepeer):'
         ui.debug('using %s\n' % self._url)
 
         self._urlopener = urlmod.opener(ui, authinfo)
-        self._requestbuilder = urlreq.request
 
     def __del__(self):
         urlopener = getattr(self, '_urlopener', None)