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