Got git-annex downloading versioned files from S3, without needing S3
credentials. This makes a S3 special remote be equally capable as a
git-annex repository exported over http, other than of course not including
the git objects.

An example of this new feature:

	AWS_SECRET_ACCESS_KEY=... AWS_ACCESS_KEY_ID=...
	git annex initremote s3 type=S3 public=yes exporttree=yes versioning=yes
	git annex export --tracking master --to s3
	git tag 1.0
	# modify some files here
	git annex sync --content s3

And then in a clone without the credentials:

	git annex enableremote s3
	git checkout 1.0
	git annex get somefile

This is nice; I only wish it were supported by other special remotes.
It seems that any special remote could be made to support it, but ones not
supporting some kind of versioning would need to store each file twice,
and many would also need each file to be uploaded to them twice. But
perhaps there are others that do have a form of versioning.
WebDAV for one has a versioning extension in RFC 3253.

Also did a final review of a patch Antoine Beaupré is working on to
backport the recent git-annex security fixes to debian oldstable,
git-annex 5.20141125. He described the backport [in his blog](https://anarc.at/blog/2018-08-31-report/):

> This time again, Haskell was nice to work with: by changing type
> configurations and APIs, the compiler makes sure that everything works out and
> there are no inconsistencies. This logic is somewhat backwards to what we are
> used to: normally, in security updates, we avoid breaking APIs at all costs.
> But in Haskell, it's a fundamental way to make sure the system is still
> coherent.

Today's work was sponsored by Trenton Cronholm [on Patreon](https://patreon.com/joeyh).
