tests/sitecustomize.py

convert: fix the RevisionSpec import in the bzr module

This import was failing (because its invalid) which was resulting in
the following tests failing with chg running:

 - test-convert-bzr.t
 - test-convert-bzr-directories.t
 - test-convert-bzr-ghosts.t
 - test-convert-bzr-treeroot.t
 - test-convert-bzr-114.t
 - test-convert-bzr-merges.t

This commit fixes the import which in turn fixes the tests.

Test Plan:
Ran the aforementioned tests with and without '--chg' option.

Differential Revision: https://phab.mercurial-scm.org/D936

from __future__ import absolute_import
import os

if os.environ.get('COVERAGE_PROCESS_START'):
    try:
        import coverage
        import uuid

        covpath = os.path.join(os.environ['COVERAGE_DIR'],
                               'cov.%s' % uuid.uuid1())
        cov = coverage.coverage(data_file=covpath, auto_data=True)
        cov._warn_no_data = False
        cov._warn_unimported_source = False
        cov.start()
    except ImportError:
        pass