hgtagsfnodescache: inherit fnode from parent when possible If a changeset does not update the content of `.hgtags`, it means it will use the same file-node (for `.hgtags`) as its parents. In this case we can directly reuse the parent's file-node. We use this property when updating the `hgtagsfnodescache` taking a faster path if we already have a cached value for the parents of the node we are looking at. Doing so provides a large performance boost when looking at a lot of fnodes, especially on repository with very large manifest: timing for `tagsmod.fnoderevs(ui, repo, repo.changelog.revs())` mercurial: (41907 revisions, 1923 files) before: 6.9 seconds after: 2.7 seconds (-54%) pypy: (96266 revisions, 5198 files) before: 80 seconds after: 20 seconds (-75%) mozilla-central: (463411 revisions, 272080 files) before: 7166.4 seconds after: 47.8 seconds (-99%, x150 speedup) On a copy of mozilla-try with about 35K heads ans 1.7M changesets, this moves the computation from many hours to a couple of minutes, making it more interesting to do a full warm up of this cache before computing tags (from a cold cache). There seems to be other performance low hanging fruits, like avoiding the use of changectx or a more revision centric logic. However, the new code is fast enough for my needs right now.