Mark some cygpath tests xfail

Two of the groups of cygpath tests in test_util.py generate tests
that fail on Cygwin. There is no easy way to still run, but xfail,
just the specific tests that fail, because the groups of tests are
generated with `@ddt` parameterization, but neither the unittest
nor pytest xfail mechanisms interact with that. If
`@pytest.mark.parametrized` were used, this could be done, but
that does not work on methods of test classes that derive from
unittest.TestCase, including those in this project that indirectly
derive from TestBase, which cannot be removed with overhauling many
tests, due to fixtures it provides such as rorepo.

So this marks too many tests as xfail, but in doing so allows test
runs to pass while still exercising and showing status on all the
tests, allowing result changes to be observed easily.

test/test_util.py

@@ -13,6 +13,7 @@
 from datetime import datetime
 
 import ddt
+import pytest
 
 from git.cmd import dashify
 from git.compat import is_win
@@ -84,13 +85,21 @@ def setup(self):
             "array": [42],
         }
 
+    @pytest.mark.xfail(
+        reason="Many return paths prefixed /proc/cygdrive instead.",
+        raises=AssertionError,
+    )
     @skipUnless(sys.platform == "cygwin", "Paths specifically for Cygwin.")
     @ddt.idata(_norm_cygpath_pairs + _unc_cygpath_pairs)
     def test_cygpath_ok(self, case):
         wpath, cpath = case
         cwpath = cygpath(wpath)
         self.assertEqual(cwpath, cpath, wpath)
 
+    @pytest.mark.xfail(
+        reason=r'2nd example r".\bar" -> "bar" fails, returns "./bar"',
+        raises=AssertionError,
+    )
     @skipUnless(sys.platform == "cygwin", "Paths specifically for Cygwin.")
     @ddt.data(
         (r"./bar", "bar"),