hidden: support passing --hidden with `serve --stdio`

This currently has no effect since the server code will filter the repository
again. However, it is clearer to put this change in its own changeset.

This will mostly impact ssh peers, clients being now able to pass --hidden when
spawning the server.

See the next changeset for details on how this is going to be used.

mercurial/dispatch.py

@@ -367,12 +367,18 b' def _runcatch(req):'
                 # shenanigans wherein a user does something like pass
                 # --debugger or --config=ui.debugger=1 as a repo
                 # name. This used to actually run the debugger.
+                nbargs = 4
+                hashiddenaccess = b'--hidden' in cmdargs
+                if hashiddenaccess:
+                    nbargs += 1
                 if (
-                    len(req.args) != 4
+                    len(req.args) != nbargs
                     or req.args[0] != b'-R'
                     or req.args[1].startswith(b'--')
                     or req.args[2] != b'serve'
                     or req.args[3] != b'--stdio'
+                    or hashiddenaccess
+                    and req.args[4] != b'--hidden'
                 ):
                     raise error.Abort(
                         _(b'potentially unsafe serve --stdio invocation: %s')