branchmap: add the tipnode (cache key) on the branchcache object Gathering data and cache key paves the way to a lot of simplification.

branchmap: store branchcache in a dedicated object Value and key of branchcache would benefit from being hold by the same object. Moreover some logic (update, write, validation) could be move on such object. The creation of this object is the first step toward this move. The result will clarify branchcache related code and hide most of the detail in the class itself. This encapsulation will greatly helps implementation of branchcache for filtered view of the repo.

branchmap: `stream_in` write remote branchcache to local one The previous code was writing it to a non existent `branchcache` attribute. We now write is to the proper `_branchcache` attribute and initialize the `_branchcachetip` at the same time. We keep writing it to disk, the previous code had this part right.

bookmarks: fix head selection for merge with two bookmarked heads A type mismatch caused the search for the other head to fail. The code is fragile, and instead it ended up using the 'first' bookmark head, but the ordering is undefined and it could thus randomly use the wrong bookmarkhead and fail with: $ hg up -q -C e@diverged $ hg merge abort: merging with a working directory ancestor has no effect

branchmap: extract updatebranchcache from repo

branchmap: extract _updatebranchcache from repo

branchmap: _updatebranchmap does not need to be filtered The `_updatebranchmap` method on repo does not need to be filtered as all callers are already handling filtering themself. The fact it is filtered may had even lead to buggy behaviors, but by chances the method make very sparse use of the repo object.

branchmap: extract read logic from repo

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.