localrepo: add separate methods for manipulating repository data This change adds new methods sjoin and sopener for accessing the following elements: - changelog - manifest - data/ - journal and undo log - repo lock This will simplify sharing this data and escaping paths

Minor tags optimization

Use file contexts in tags code

filecontext: report False for missing files

Only look up tags and branches as a last resort

If we can't write the branch cache, fail quietly.