rev-branch-cache: move the code in a dedicated module
The branchmap module is getting huge and the rev branch cache is fully
independent, lets move it elsewhere.
= Mercurial XXX =
== New Features ==
== Default Format Change ==
These changes affect newly created repositories (or new clones) done with
Mercurial XXX.
== New Experimental Features ==
== Bug Fixes ==
== Backwards Compatibility Changes ==
== Internal API Changes ==
== Miscellaneous ==