Problem:    The default conceal character is documented to be a space but it's
            initially a dash. (Christian Brabandt)
Solution:   Make the intial value a space.

https://github.com/vim/vim/commit/4a427106954857c6f84c0b0dc07725e302a433c6

Closes #3754

tutor: Update vim-tutor to 0.2.1

Also, some tweaks based on input by @fdinoff on gitter.

shada: Do not save unlisted and quickfix buffers

[RFC] doc: Remove todo.txt

It never did anything (see `:h hidden-options`).

It's not documentation in the same sense as the majority of files in
runtime/doc, so is of little use to most users and should probably not
be installed alongside the "real" documentation.

It may be full of good ideas, but it's also full of things which are no
longer applicable to Nvim, such as references to platforms we no longer
support (e.g., MS-DOS), Vi compatibility, Autoconf, the Perl interface,
etc.

If someone is looking for things to fix or improve, the GitHub issue
tracker should prove useful, and, unlike todo.txt, the issues are
generally much more relevant to Nvim.  Besides all that, removing
todo.txt makes porting runtime patches a bit easier.

refs #2911, discussed briefly in #2608 and #2553

All `{not in Vi}' tags have been removed, so the first sentence is no
longer true.  As for the second sentence, the header ("Differences
between Vim and Vi") is already self-explanatory, so it can be removed
as well.

We don't care about Vi here, only Vim.