branchmap: extract write logic from localrepo

branchmap: create a mercurial.branchmap module This is the foundation stone for an extraction of branches map logic from local repository class. Most of the branches map logic have very few caller and therefor does not fit in the current criteria for code held by the localrepo class. Important change will be made to this code in relation with revision filtering. So we extract it in a dedicated module before adding additional complexity. Follow up commit do the actual code movement.

bundlerepo: use _cacheabletip mechanism in bundlerepo Instead of preventing any cache write we allow writing cache for all content of the original repo. The motivation for this change is to drop the custom _writebranchcache of bundlerepo to help extraction of the branchmap logic out of localrepo.

branchmap: merge _branchtags into updatebranchcache Now that nobody overwrite it, there is no reasons for `_branchtags` to remains separated from `updatebranchcache`.

branchmap: factorise access to changelog in updatebranchcache This prepares merge of `updatebranchcache` and `_branchtags`.

branchmap: simplify _branchtags using a new _cacheabletip method The current _branchtags method is a remnant of an older, much larger function. Its only remaining role is to help MQ to alter the version of branchcache we store on disk. As MQ mutates the repository it ensures persistent cache does not contain anything it is likely to alter. This changeset makes explicit the stable vs volatile part of repository and reduces the MQ specific code to the computation of this limit. The main _branchtags code now handles this possible limit in all cases. This will help to extract the branchmap logic from the repository. The new code of _branchtags is a bit duplicated, but as I expect major refactoring of this section I'm not keen to setup factorisation function here.