largefiles: overridematch() should replace the file path instead of extending (issue3934)

revlog: handle hidden revs in _partialmatch (issue3979) Looking up hidden prefixes could cause a no node exception Looking up unique non-hidden prefixes could be ambiguous

fileset: handle underbar in symbols This was documented, but not implemented.

heads: modernize documentation (issue3992) The old docs emphasized topological heads rather than branch heads and incorrectly defined branch heads as not having children rather than descendants.