Add Windows support

[christian-monch]

Windows is currently not supported because:

1. It seems that git-annex does not work with git-worktree on Windows.
2. Tests rely on commands not generally available on Windows, e.g. echo.

Recreate the Windows error

The following commands in a PowerShell console in Windows demonstrate the git-annex issue, i.e. it is not possible to the content of the annexed file a.txt in the worktree.

> datalad create test-ds
> cd test-ds
> write-output some_content > a.txt
> datalad save
> git worktree add wt-1
> cd wt-1
> cat .\a.txt
/annex/objects/MD5E-s30--5e21038aabfc0c59d172ea1d7a64cc67.txt
> datalad get a.txt
get(error): a.txt (file) [not available]
[{'action': 'get',
  'annexkey': 'MD5E-s30--5e21038aabfc0c59d172ea1d7a64cc67.txt',
  'message': 'not available',
  'path': 'C:\\Users\\test-user\\tmp\\test-ds\\wt-1\\a.txt',
  'refds': 'C:\\Users\\test-user\\tmp\\test-ds\\wt-1',
  'status': 'error',
  'type': 'file'}])