tags: cache `repo.changelog` access when checking tags nodes The tags reading process checks if the nodes referenced in tags exist. Caching the access to `repo.changelog` provides a large speedup for repositories with many tags. running `hg perftags` in a large private repository before: ! wall 0.393464 comb 0.390000 user 0.330000 sys 0.060000 (median of 25) after: ! wall 0.267711 comb 0.270000 user 0.210000 sys 0.060000 (median of 38)