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%)