http2: send an extra header to signal a non-broken client

Some proxies strip the expect header because they forward requests in
a non-compliant way or as a way to defend against bogus clients.

mercurial/hgweb/hgweb_mod.py

@@ -129,8 +129,9 b' class hgweb(object):'
                 # A client that sends unbundle without 100-continue will
                 # break if we respond early.
                 if (cmd == 'unbundle' and
-                    req.env.get('HTTP_EXPECT',
-                                '').lower() != '100-continue'):
+                    (req.env.get('HTTP_EXPECT',
+                                 '').lower() != '100-continue') or
+                    req.env.get('X-HgHttp2', '')):
                     req.drain()
                 req.respond(inst, protocol.HGTYPE)
                 return '0\n%s\n' % inst.message

mercurial/httprepo.py

@@ -78,6 +78,7 b' class httprepository(wireproto.wirerepos'
 
         if data and self.ui.configbool('ui', 'usehttp2', False):
             headers['Expect'] = '100-Continue'
+            headers['X-HgHttp2'] = '1'
 
         self.ui.debug("sending %s command\n" % cmd)
         q = [('cmd', cmd)]