hgweb: fix trust of templates path (BC)

Long ago we disabled trust of the templates path with a comment
describing the (insecure) behavior before the change. At some later
refactor, the code was apparently changed back to match the comment,
unaware that the intent of the comment was to describe the behavior to
avoid.

This change disables the trust and updates the comment to explicitly
say not only what the old problem was, but also that it was in fact a
problem and the action taken to prevent it.

Impact: prior to this change, if you had a UNIX-based hgweb server
where users can write hgrc files, those users could potentially read
any file readable by the web server.

This is marked as a backwards compatibility issue because people may
have configured templates without proper trust settings. Issue spotted
by Greg Szorc.

mercurial/hgweb/hgweb_mod.py

@@ -89,9 +89,10 b' class hgweb(object):'
         self.reponame = name
         self.archives = 'zip', 'gz', 'bz2'
         self.stripecount = 1
-        # a repo owner may set web.templates in .hg/hgrc to get any file
-        # readable by the user running the CGI script
-        self.templatepath = self.config('web', 'templates')
+        # we use untrusted=False to prevent a repo owner from using
+        # web.templates in .hg/hgrc to get access to any file readable
+        # by the user running the CGI script
+        self.templatepath = self.config('web', 'templates', untrusted=False)
         self.websubtable = self.loadwebsub()
 
     # The CGI scripts are often run by a user different from the repo owner.