context: make warning message for hidden revision extensible

Extensions might want to create new filternames and change what revisions
are considered hidden or shown. This is the case for inhibit that enables
direct access to hidden hashes with the visible-directaccess-nowarn filtername.
By using startswith instead of a direct comparison with 'visible' we
allow extensions to do that and not work directly on the 'visible' filtername
used by core.

mercurial/context.py

@@ -459,7 +459,7 b' class changectx(basectx):'
                 pass
         except (error.FilteredIndexError, error.FilteredLookupError,
                 error.FilteredRepoLookupError):
-            if repo.filtername == 'visible':
+            if repo.filtername.startswith('visible'):
                 msg = _("hidden revision '%s'") % changeid
                 hint = _('use --hidden to access hidden revisions')
                 raise error.FilteredRepoLookupError(msg, hint=hint)