tags-fnode-cache: do not repeatedly open the filelog in a loop

While getting multiple hgtagsfnodecache entries, we were opening (and closing)
the `.hgtags` filelog for each iteration. The meant repeatedly reading and
parsing the version same information from disk. A quite costly operation.

We no longer do this, leading to a sizable improvement in `hg debugupdatecache`
run for an already warm repositories.

### data-env-vars.name           = mercurial-2018-08-01-zstd-sparse-revlog
  # benchmark.name               = debug-update-cache
  # benchmark.variants.pre-state = warm
before:  1.711778 seconds
after:   0.213229 seconds  (-87.54%)
  # data-env-vars.name           = pypy-2018-08-01-zstd-sparse-revlog
before:  4.010817 seconds
after:   0.381141 seconds  (-90.50%)
  # data-env-vars.name           = netbeans-2018-08-01-zstd-sparse-revlog
before: 13.574141
after:   1.023007 seconds  (-92.46%)
  # data-env-vars.name           = mozilla-central-2018-08-01-zstd-sparse-revlog
before: 18.884656
after:   1.465735 seconds  (-92.24%)
  # data-env-vars.name           = mozilla-try-2019-02-18-zstd-sparse-revlog
before: 88.924823
after:   6.511771 seconds  (-92.68%)

mercurial/tags.py

@@ -491,11 +491,14 b' def _getfnodes(ui, repo, nodes):'
     cachefnode = {}
     validated_fnodes = set()
     unknown_entries = set()
+
+    flog = None
     for node in nodes:
         fnode = fnodescache.getfnode(node)
-        flog = repo.file(b'.hgtags')
         if fnode != repo.nullid:
             if fnode not in validated_fnodes:
+                if flog is None:
+                    flog = repo.file(b'.hgtags')
                 if flog.hasnode(fnode):
                     validated_fnodes.add(fnode)
                 else: