Hide Git submodules from plugin managers

# Hack: because most plugin managers will try to automatically download

# submodules I have renamed the `.gitmodules` file.  Contributors will have to

# symlink the `.submodules` file to `.gitmodules` before they can use submodule

# commands

/.gitmodules

# Tags are generated by Vim automatically

/doc/tags

/test/xdg/local/state

my knowledge there is no way of fixing this without changing Vim.

Contributing

############

TL;DR: Run these steps first if you want to run tests:

.. code:: sh

   ln -s .submodules .gitmodules

   git submodule init

   git submodule updated --checkout

To provide some context, there are dependencies needed to run tests.  These are

included as Git submodules.  The problem is that a number of package managers

will download submodules by default, which means that users will end up with a

bunch of stuff installed that they don't want.  There is no way to tell Git to

use a different file, so we have to create a symbolic link instead if we

actually do want to download these submodules.

License

#######