fileset: handle underbar in symbols

This was documented, but not implemented.

--HG--
branch : stable
extra : amend_source : 03a3a7150bfec1fd6a2646574b005ec2657e22fb
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

--HG--
branch : stable
largefiles: overridematch() should replace the file path instead of extending (issue3934)

--HG--
branch : stable