cygwin, #533: polish abs-paths in git add commands

+ Modify TCs - no main-code changes.
+ FIXed:
  + `TestSubmodule.test_git_submodules_and_add_sm_with_new_commit()`
  + TestDiff.test_diff_with_staged_file()
  
- Cygwin TCs failing:
  - PY2: err: 11, fail: 2
  - PY3: err: 10, fail: 2

Author: ankostis

Diff for: git/test/test_diff.py

@@ -25,6 +25,7 @@
     NULL_TREE,
 )
 import ddt
+from git.cmd import Git
 
 
 @ddt.ddt
@@ -56,7 +57,7 @@ def test_diff_with_staged_file(self, rw_dir):
         fp = os.path.join(rw_dir, 'hello.txt')
         with open(fp, 'w') as fs:
             fs.write("hello world")
-        r.git.add(fp)
+        r.git.add(Git.polish_url(fp))
         r.git.commit(message="init")
 
         with open(fp, 'w') as fs:

Diff for: git/test/test_submodule.py

@@ -705,7 +705,7 @@ def test_git_submodules_and_add_sm_with_new_commit(self, rwdir):
         fp = osp.join(smm.working_tree_dir, 'empty-file')
         with open(fp, 'w'):
             pass
-        smm.git.add(fp)
+        smm.git.add(Git.polish_url(fp))
         smm.git.commit(m="new file added")
 
         # submodules are retrieved from the current commit's tree, therefore we can't really get a new submodule

Diff for: git/util.py

@@ -264,6 +264,7 @@ def _cygexpath(drive, path):
 
 
 def cygpath(path):
+    """Use :meth:`git.cmd.Git.polish_url()` instead, that works on any environment."""
     if not path.startswith(('/cygdrive', '//')):
         for regex, parser, recurse in _cygpath_parsers:
             match = regex.match(path)