Like --hide-missing the branch does not get updated when content
availability changes.
Seems to basically work, but sync does not update it yet.
Also, when a file is present and so unlocked, git mv followed by
git-annex sync results in the basis branch being updated to contain the
file with the new name, unlocked. This seems different than what
happens in an adjusted unlocked branch, where the commit propigates back
locked. Probably the reverse adjustment code needs to be improved to
handle this case.
