tags-fnode-cache: skip building a changectx in getfnode stable

tags-fnode-cache: skip building a changectx in getfnode

Building a changectx object is costly, doing it just to retrieve the revision
number is suboptimal. Directly fetching the revision number from the changelog
provide a sizeable speedup to `hg debugupdatecache`.

### data-env-vars.name           = mercurial-2018-08-01-zstd-sparse-revlog
  # benchmark.name               = debug-update-cache
  # benchmark.variants.pre-state = warm
before: 0.213229 seconds
after:  0.165577 seconds  (-22.35%)
  # data-env-vars.name           = mercurial-filtered-2019-11-22-zstd-sparse-revlog
before: 1.200383 seconds
after:  1.071618 seconds  (-10.73%)
  # data-env-vars.name           = mozilla-central-2018-08-01-zstd-sparse-revlog
before: 1.465735 seconds
after:  0.923128 seconds  (-37.02%)
  # data-env-vars.name           = mozilla-try-2019-02-18-zstd-sparse-revlog
before: 6.511771 seconds
after:  4.507316 seconds  (-30.78%)
  # data-env-vars.name           = netbeans-2018-08-01-zstd-sparse-revlog
before: 1.023007 seconds
after:  0.645026 seconds  (-36.95%)
  # data-env-vars.name           = pypy-2018-08-01-zstd-sparse-revlog
before: 0.381141 seconds
after:  0.268654 seconds  (-29.51%)

--- a/mercurial/tags.py	Sat Nov 12 02:38:26 2022 +0100
+++ b/mercurial/tags.py	Sat Nov 12 02:38:53 2022 +0100
@@ -761,8 +761,7 @@
         if node == self._repo.nullid:
             return node
 
-        ctx = self._repo[node]
-        rev = ctx.rev()
+        rev = self._repo.changelog.rev(node)
 
         self.lookupcount += 1