cygwin, #533: FIX submodules detection (~10TCs fixed)

+ Decygpath sm's `.git` file contents.

- Cygwin TCs failing:
  - PY2: err: 2, fail: 2
  - PY3: err: 2, fail: 2

Author: ankostis

Diff for: git/repo/base.py

@@ -109,7 +109,7 @@ def __init__(self, path=None, odbt=DefaultDBType, search_parent_directories=Fals
         :raise InvalidGitRepositoryError:
         :raise NoSuchPathError:
         :return: git.Repo """
-        if path and Git.is_cygwin():
+        if Git.is_cygwin():
             path = decygpath(path)
 
         epath = _expand_path(path or os.getcwd())

Diff for: git/repo/fun.py

@@ -6,13 +6,14 @@
 from git.exc import WorkTreeRepositoryUnsupported
 from git.objects import Object
 from git.refs import SymbolicReference
-from git.util import hex_to_bin, bin_to_hex
+from git.util import hex_to_bin, bin_to_hex, decygpath
 from gitdb.exc import (
     BadObject,
     BadName,
 )
 
 import os.path as osp
+from git.cmd import Git
 
 
 __all__ = ('rev_parse', 'is_git_dir', 'touch', 'find_git_dir', 'name_to_object', 'short_to_long', 'deref_tag',
@@ -59,6 +60,9 @@ def find_git_dir(d):
     else:
         if content.startswith('gitdir: '):
             path = content[8:]
+            if Git.is_cygwin():
+                ## Cygwin creates submodules prefixed with `/cygdrive/...` suffixes.
+                path = decygpath(path)
             if not osp.isabs(path):
                 path = osp.join(osp.dirname(d), path)
             return find_git_dir(path)