branchcache: introduce revbranchcache for caching of revision branch names It is expensive to retrieve the branch name of a revision. Very expensive when creating a changectx and calling .branch() every time - slightly less when using changelog.branchinfo(). Now, to speed things up, provide a way to cache the results on disk in an efficient format. Each branchname is assigned a number, and for each revision we store the number of the corresponding branch name. The branch names are stored in a dedicated file which is strictly append only. Branch names are usually reused across several revisions, and the total list of branch names will thus be so small that it is feasible to read the whole set of names before using the cache. It will however do that it might be more efficient to use the changelog for retrieving the branch info for a single revision. The revision entries are stored in another file. This file is usually append only, but if the repository has been modified, the file will be truncated and the relevant parts rewritten on demand. The entries for each revision are 8 bytes each, and the whole revision file will thus be 1/8 of 00changelog.i. Each revision entry contains the first 4 bytes of the corresponding node hash. This is used as a check sum that always is verified before the entry is used. That check is relatively expensive but it makes sure history modification is detected and handled correctly. It will also detect and handle most revision file corruptions. This is just a cache. A new format can always be introduced if other requirements or ideas make that seem like a good idea. Rebuilding the cache is not really more expensive than it was to run for example 'hg log -b branchname' before this cache was introduced. This new method is still unused but promise to make some operations several times faster once it actually is used. Abandoning Python 2.4 would make it possible to implement this more efficiently by using struct classes and pack_into. The Python code could probably also be micro optimized or it could be implemented very efficiently in C where it would be easy to control the data access.