CONTRIBUTING

tests: convert the `root` arg of matchmod.match() to local path separators

This fixes tests that broke with 8b1a9ba375e5, complaining that "X not under
root /repo".  The vast majority of real uses are to pass `repo.root`, which is
normalized by `wdirvfs.base` being set to the result of `os.path.realpath()`.

Failure to convert looks like this:

  --- c:/Users/Matt/hg/tests/test-match.py.out
  +++ c:/Users/Matt/hg/tests/test-match.py.err
  @@ -0,0 +1,48 @@
  +ERROR: testVisitchildrensetGlob (__main__.IncludeMatcherTests)
  +
  +Traceback (most recent call last):
  +  File "c:\Users\Matt\hg\tests\test-match.py", line 180, in testVisitchildrensetGlob
  +    m = matchmod.match(b'/repo', b'', include=[b'glob:dir/z*'])
  +  File "c:\Users\Matt\hg\mercurial\match.py", line 271, in match
  +    kindpats = normalize(include, b'glob', root, cwd, auditor, warn)
  +  File "c:\Users\Matt\hg\mercurial\match.py", line 322, in _donormalize
  +    pat = pathutil.canonpath(root, cwd, pat, auditor=auditor)
  +  File "c:\Users\Matt\hg\mercurial\pathutil.py", line 251, in canonpath
  +    _(b"%s not under root '%s'") % (myname, root), hint=hint
  +Abort: dir/z* not under root '/repo'
  +ERROR: testVisitdirGlob (__main__.IncludeMatcherTests)

Differential Revision: https://phab.mercurial-scm.org/D7724

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