dispatch: extract common logic for handling ParseError The way ParseError is handled at two different places in dispatch.py is the same. Move common logic into _formatparse.

chgserver: resolve relative path before sending via system channel The chgserver may have a different cwd from the client because of the side effect of "--cwd" and other possible os.chdir done by extensions. Therefore relative paths can be misunderstood by the client. This patch solves it by expanding relative cwd path to absolute one before sending them via the 'S' channel. It can help chg to pass a testcase in test-alias.t later.

mercurial: use pure Python module policy on Python 3 The C extensions don't yet work with Python 3. Let's minimize the work required to get Mercurial running on Python 3 by always using the pure Python module policy on Python 3.