handle Annex.Branch.files with read-only unmerged git-annex branches

handle Annex.Branch.files with read-only unmerged git-annex branches

It would be difficult to make Annex.Branch.files query the unmerged
git-annex branches. Might be possible, similar to what was discussed in
7f6b2ca49cd6f6b6dd6cc9e96d0232a213a82e63 but again I decided to make it
not do anything in that situation to start with before adding such a
complicated thing.

git-annex info uses it when getting info about a repostory. The choices
were to make that fail with an error, or display the info it can, and
change the output slightly for the bits of info it cannot access. While
that is a behavior change, and I want to avoid any behavior changes due
to unmerged git-annex branches in a read-only repo, displaying a message
that is not a number seems unlikely to break anything that was consuming
a number, any worse than throwing an exception would. Probably.

Also git-annex unused --from origin is made to throw an error, but
it would fail later anyway when trying to write to the unused log files.

Sponsored-by: Dartmouth College's Datalad project