improve handling of unmerged git-annex branches in readonly repo

git-annex info was displaying a message that didn't make sense in
context.

In calcRepoSizes, it seems better to return the information from the
git-annex branch, rather than giving up. Especially since balanced
preferred content uses it, and we can't just give up evaluating a
preferred content expression if git-annex is to be usable in such a
readonly repo.

Commit 6d7ecd9e5d nobly wanted git-annex
to behave the same with such unmerged branches as it does when it can
merge them. But for the purposes of preferred content, it seems to me
there's a sense that such an unmerged branch is the same as a remote we
have not pulled from. The balanced preferred content will either way
operate under outdated information, and so make not the best choices.

Database/ImportFeed.hs

@@ -188,8 +188,8 @@ updateFromLog db@(ImportFeedDbHandle h) (oldtree, currtree)
 	-- is faster than diffing from the empty tree
 	-- and looking up every log file.
 	scanbranch = Annex.Branch.overBranchFileContents toscan goscan >>= \case
-		Just () -> return ()
-		Nothing -> scandiff
+		Annex.Branch.NoUnmergedBranches () -> return ()
+		Annex.Branch.UnmergedBranches () -> scandiff
 	
 	toscan f
 		| isUrlLog f = Just ()