fileset, revset: do not use global parser object for thread safety

parse() cannot be called at the same time because a parser object keeps its
states.  This is no problem for command-line hg client, but it would cause
strange errors in multi-threaded hgweb.

Creating parser object is not too expensive.

original:
% python -m timeit -s 'from mercurial import revset' 'revset.parse("0::tip")'
100000 loops, best of 3: 11.3 usec per loop

thread-safe:
% python -m timeit -s 'from mercurial import revset' 'revset.parse("0::tip")'
100000 loops, best of 3: 13.1 usec per loop

--HG--
branch : stable