hgweb: remove support for POST form data (BC)

hgweb: remove support for POST form data (BC)

Previously, we called out to cgi.parse(), which for POST requests
parsed multipart/form-data and application/x-www-form-urlencoded
Content-Type requests for form data, combined it with query string
parameters, returned a union of the values.

As far as I know, nothing in Mercurial actually uses this mechanism
to submit data to the HTTP server. The wire protocol has its own
mechanism for passing parameters. And the web interface only does
GET requests. Removing support for parsing POST data doesn't break
any tests.

Another reason to not like this feature is that cgi.parse() may
modify the QUERY_STRING environment variable as a side-effect.
In addition, it merges both POST data and the query string into
one data structure. This prevents consumers from knowing whether
a variable came from the query string or POST data. That can matter
for some operations.

I suspect we use cgi.parse() because back when this code was
initially implemented, it was the function that was readily
available. In other words, I don't think there was conscious
choice to support POST data: we just got it because cgi.parse()
supported it.

Since nothing uses the feature and it is untested, let's remove
support for parsing POST form data. We can add it back in easily
enough if we need it in the future.

.. bc::

   Hgweb no longer reads form data in POST requests from
   multipart/form-data and application/x-www-form-urlencoded
   requests. Arguments should be specified as URL path components
   or in the query string in the URL instead.

Differential Revision: https://phab.mercurial-scm.org/D2774

--- a/mercurial/hgweb/request.py	Sat Mar 10 11:06:13 2018 -0800
+++ b/mercurial/hgweb/request.py	Sat Mar 10 11:07:53 2018 -0800
@@ -8,7 +8,6 @@
 
 from __future__ import absolute_import
 
-import cgi
 import errno
 import socket
 import wsgiref.headers as wsgiheaders
@@ -258,15 +257,12 @@
         self.multiprocess = wsgienv[r'wsgi.multiprocess']
         self.run_once = wsgienv[r'wsgi.run_once']
         self.env = wsgienv
-        self.form = normalize(cgi.parse(inp,
-                                        self.env,
-                                        keep_blank_values=1))
+        self.req = parserequestfromenv(wsgienv, inp)
+        self.form = normalize(self.req.querystringdict)
         self._start_response = start_response
         self.server_write = None
         self.headers = []
 
-        self.req = parserequestfromenv(wsgienv, inp)
-
     def respond(self, status, type, filename=None, body=None):
         if not isinstance(type, str):
             type = pycompat.sysstr(type)