debug-discovery: properly apply remote filtering in "old" mode stable

debug-discovery: properly apply remote filtering in "old" mode

Before this change using `--remote-as-revs` with `--old` had no effect and
everything was considered as "common", which is really not what we intended.

--- a/mercurial/debugcommands.py	Tue Jul 26 04:54:59 2022 +0200
+++ b/mercurial/debugcommands.py	Tue Jul 26 04:56:29 2022 +0200
@@ -1276,6 +1276,9 @@
             if not util.safehasattr(remote, b'branches'):
                 # enable in-client legacy support
                 remote = localrepo.locallegacypeer(remote.local())
+                if remote_revs:
+                    r = remote._repo.filtered(b'debug-discovery-remote-filter')
+                    remote._repo = r
             common, _in, hds = treediscovery.findcommonincoming(
                 repo, remote, force=True, audit=data
             )