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operate under outdated information, and so make not the best choices.

Limit.hs

@@ -600,10 +600,12 @@ limitFullyBalanced mu getgroupmap groupname = Right $ MatchFiles
 		maxsizes <- getMaxSizes
 		-- XXX do not calc this every time!
 		sizemap <- calcRepoSizes
+		let keysize = fromMaybe 0 (fromKey keySize key)
 		let hasspace u = case (M.lookup u maxsizes, M.lookup u sizemap) of
 			(Just (MaxSize maxsize), Just (RepoSize reposize)) ->
-				reposize + fromMaybe 0 (fromKey keySize key) 
-					<= maxsize
+				if maybe False (`S.member` notpresent) mu
+					then reposize <= maxsize
+					else reposize + keysize <= maxsize
 			_ -> True
 		let candidates = S.filter hasspace groupmembers
 		return $ if S.null candidates