Fix broken worktree path with submodules on Windows

[msiemens]

On Windows, repo.create_submodule(...) failed because git didn't recognize the worktree path set in .git/modules/sub/config (fatal: bad config file line 6 in ./config). This resulted in the following traceback:

Traceback (most recent call last):
  File "F:\Dokumente\Coding\python\PyGitUp\tests\test_submodules.py", line 42, in setup
    repo.create_submodule('sub', 'sub', master_path)
  File "F:\Dokumente\Coding\python\.virtualenv\PyGitUp\lib\site-packages\git\repo\base.py", line 295, in create_submodule
    return Submodule.add(self, *args, **kwargs)
  File "F:\Dokumente\Coding\python\.virtualenv\PyGitUp\lib\site-packages\git\objects\submodule\base.py", line 417, in add
    sm.binsha = mrepo.head.commit.binsha
  File "F:\Dokumente\Coding\python\.virtualenv\PyGitUp\lib\site-packages\git\refs\symbolic.py", line 182, in _get_commit
    obj = self._get_object()
  File "F:\Dokumente\Coding\python\.virtualenv\PyGitUp\lib\site-packages\git\refs\symbolic.py", line 175, in _get_object
    return Object.new_from_sha(self.repo, hex_to_bin(self.dereference_recursive(self.repo, self.path)))
  File "F:\Dokumente\Coding\python\.virtualenv\PyGitUp\lib\site-packages\git\objects\base.py", line 65, in new_from_sha
    oinfo = repo.odb.info(sha1)
  File "F:\Dokumente\Coding\python\.virtualenv\PyGitUp\lib\site-packages\git\db.py", line 40, in info
    hexsha, typename, size = self._git.get_object_header(bin_to_hex(sha))
  File "F:\Dokumente\Coding\python\.virtualenv\PyGitUp\lib\site-packages\git\cmd.py", line 866, in get_object_header
    return self.__get_object_header(cmd, ref)
  File "F:\Dokumente\Coding\python\.virtualenv\PyGitUp\lib\site-packages\git\cmd.py", line 855, in __get_object_header
    return self._parse_object_header(cmd.stdout.readline())
  File "F:\Dokumente\Coding\python\.virtualenv\PyGitUp\lib\site-packages\git\cmd.py", line 817, in _parse_object_header
    raise ValueError("SHA could not be resolved, git returned: %r" % (header_line.strip()))
ValueError: SHA could not be resolved, git returned: ''

This PR makes _write_git_file_and_module_config convert the worktree path to the linux format (forward slashes) which git recognizes.

[Byron]

Thank you.
Would you mind fixing this issue reported by flake8 ?

$ flake8
./git/objects/submodule/base.py:296:121: E501 line too long (123 > 120 characters)

Then I'd be glad to merge and release it this week. Thanks again.

[msiemens]

Sure! I've updated the PR now. The Travis tests failed, but it doesn't seem to be related to this change.

[Byron]

Indeed, the other travis issue will be on my plate.
Thank you !

[msiemens]

That was quick! Thanks, @Byron!