handle overBranchFileContents with read-only unmerged git-annex branches

handle overBranchFileContents with read-only unmerged git-annex branches

This makes --all error out in that situation. Which is better than
ignoring information from the branches.

To really handle the branches right, overBranchFileContents would need
to both query all the branches and union merge file contents
(or perhaps not provide any file content), as well as diffing between
branches to find files that are only present in the unmerged branches.
And also, it would need to handle transitions..

Sponsored-by: Dartmouth College's Datalad project