tests/sitecustomize.py

match: teach diffmatcher.visitdir() to return 'all' if possible

This patch teaches differencematcher.visitdir() to return 'all' when
m1.visitdir() returns 'all' and m2 does not matches.

Before this patch, from a differencematcher.visitdir(), we always returned
either True or False. We never returned 'all' even when we can. This causes
problem when m1 and m2 of a differencematcher are themselves differencematcher.
In that case, we try to check:

`if self._m2_.visitdir(dir) == 'all'`

which will never be 'all' even though it can be.

This leads to iterating over a lot of sub-directory manifest, even though we
don't want to while extending a narrow clone. I am yet to measure the impact of
this but calculating manifest was taking ~50-60 seconds, so this should
definitely save some of time there.

Differential Revision: https://phab.mercurial-scm.org/D5814

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