wireprotoserver: check if command available before calling it

wireprotoserver: check if command available before calling it

The previous behavior was just plain wrong. I have no clue how it
landed. My guess is a merge conflict resolution gone wrong on my
end a few weeks ago.

Differential Revision: https://phab.mercurial-scm.org/D2716

--- a/mercurial/wireprotoserver.py	Tue Mar 06 02:43:17 2018 -0600
+++ b/mercurial/wireprotoserver.py	Tue Mar 06 15:02:53 2018 -0800
@@ -235,14 +235,14 @@
         for chunk in gen:
             yield chunk
 
-    rsp = wireproto.dispatch(repo, proto, cmd)
-
     if not wireproto.commands.commandavailable(cmd, proto):
         req.respond(HTTP_OK, HGERRTYPE,
                     body=_('requested wire protocol command is not available '
                            'over HTTP'))
         return []
 
+    rsp = wireproto.dispatch(repo, proto, cmd)
+
     if isinstance(rsp, bytes):
         req.respond(HTTP_OK, HGTYPE, body=rsp)
         return []