Like git, git-annex hangs on to every old version of a file (by default),
so you can make tags and branches, and can check them out later to look at
the old files.

	$ git tag 1.0
	$ rm -f my_cool_big_file
	$ git commit -m deleted
	$ git checkout 1.0
	$ cat my_cool_big_file
	yay! old version still here

Of course, when you `git checkout` an old branch, some old versions of
files may not be locally available, and may be stored in some other
repository. You can use `git annex get` to get them as usual.
