Normalizes path for git and git submodules

`_git_ls_files()` returns file paths with forward slashes. For files
in submodules, add_prefix_to_each() is called to prefix all the
submodule files with the relative path to the project root. On Windows,
the relative path contains backslashes. Since not every file in the
project is in a submodule, the combined list of files ended up with
some files that use forward slashes and others that use backslashes.

For project structures where the submodules share a path segment with
the project source, this caused `add_directories()` to create duplicate
directory entries in the list of files. The duplicate entries would
result in a traceback with a WindowsError when `os.mkdir()` was called
to create the same directory a second time.

See mgedmin#61 for more details.

This commit replaces the backslashes with forward slashes in the
relative path, eliminating the duplicate directory entries.

check_manifest.py

@@ -302,7 +302,7 @@ def get_versioned_files(cls):
         files = cls._git_ls_files()
         submodules = cls._list_submodules()
         for subdir in submodules:
-            subdir = os.path.relpath(subdir)
+            subdir = os.path.relpath(subdir).replace(os.path.sep, '/')
             files += add_prefix_to_each(subdir, cls._git_ls_files(subdir))
         return add_directories(files)