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%)
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
end_of_line = lf
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true
end_of_line = lf
[*.t]
indent_size = 2
indent_style = space
trim_trailing_whitespace = false
end_of_line = lf