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operate under outdated information, and so make not the best choices.

Logs/Location.hs

@@ -219,18 +219,18 @@ loggedKeysFor' u = loggedKeys' isthere
 		return there
 
 {- This is much faster than loggedKeys. -}
-overLocationLogs :: v -> (Key -> [UUID] -> v -> Annex v) -> Annex v
+overLocationLogs :: v -> (Key -> [UUID] -> v -> Annex v) -> Annex (Annex.Branch.UnmergedBranches v)
 overLocationLogs v = overLocationLogs' v (flip const)
 
 overLocationLogs'
 	 :: v 
 	-> (Annex (Maybe (Key, RawFilePath, Maybe L.ByteString)) -> Annex v -> Annex v)
         -> (Key -> [UUID] -> v -> Annex v)
-        -> Annex v
+        -> Annex (Annex.Branch.UnmergedBranches v)
 overLocationLogs' iv discarder keyaction = do
 	config <- Annex.getGitConfig
 	clusters <- getClusters
-		
+	
 	let getk = locationLogFileKey config
 	let go v reader = reader >>= \case
 		Just (k, f, content) -> discarder reader $ do
@@ -245,9 +245,7 @@ overLocationLogs' iv discarder keyaction = do
 				)
 		Nothing -> return v
 
-	Annex.Branch.overBranchFileContents getk (go iv) >>= \case
-		Just r -> return r
-		Nothing -> giveup "This repository is read-only, and there are unmerged git-annex branches, which prevents operating on all keys. (Set annex.merge-annex-branches to false to ignore the unmerged git-annex branches.)"
+	Annex.Branch.overBranchFileContents getk (go iv)
 
 -- Cannot import Logs.Cluster due to a cycle.
 -- Annex.clusters gets populated when starting up git-annex.