tags: extract .hgtags filenodes cache to a standalone file Resolution of .hgtags filenodes values has historically been a performance pain point for large repositories, where reading individual manifests can take over 100ms. Multiplied by hundreds or even thousands of heads and resolving .hgtags filenodes becomes a performance issue. This patch establishes a standalone cache file holding the .hgtags filenodes for each changeset. After this patch, the .hgtags filenode for any particular changeset should only have to be computed once during the lifetime of the repository. The introduced hgtagsfnodes1 cache file is modeled after the rev branch cache: the cache is effectively an array of entries consisting of a changeset fragment and the filenode for a revision. The file grows in proportion to the length of the repository (24 bytes per changeset) and is truncated when the repository is stripped. The file is not written unless tag info is requested and tags have changed since last time. This patch partially addresses issue4550. Future patches will split the "tags" cache file into per-filter files and will refactor the cache format to not capture the .hgtags fnodes, as these are now stored in the hgtagsfnodes1 cache. This patch is capable of standing alone. We should not have to wait on the tags cache filter split and format refactor for this patch to land.