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.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.