match: normalize `cwd` early

By having cwd in absolute form, we won't have to adjust it when
passing it to subrepo matchers. This will matter for a coming patch.

Differential Revision: https://phab.mercurial-scm.org/D7650

mercurial/match.py

@@ -229,6 +229,7 b' def match('
     False
     """
     assert os.path.isabs(root)
+    cwd = util.normpath(os.path.join(root, cwd))
     normalize = _donormalize
     if icasefs:
         dirstate = ctx.repo().dirstate