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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.

Add test for invalid branch cache (fixed by 27ebe4efe98e)

invalidate the branch cache when sanity check fails

Testing the new changegroupsubset remote function in the ssh and http protocols.