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
7f6b2ca49c 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

Command/Info.hs

@@ -1,6 +1,6 @@
 {- git-annex command
  -
- - Copyright 2011-2020 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -370,13 +370,17 @@ local_annex_size = simpleStat "local annex size" $
 
 -- "remote" is in the name for JSON backwards-compatibility
 repo_annex_keys :: UUID -> Stat
-repo_annex_keys u = stat "remote annex keys" $ json show $
-	countKeys <$> cachedRemoteData u
+repo_annex_keys u = stat "remote annex keys" $ \d ->
+	cachedRemoteData u >>= \case
+		Right rd -> json show (pure (countKeys rd)) d
+		Left n-> json id (pure n) d
 
 -- "remote" is in the name for JSON backwards-compatibility
 repo_annex_size :: UUID -> Stat
 repo_annex_size u = simpleStat "remote annex size" $
-	showSizeKeys =<< cachedRemoteData u
+	cachedRemoteData u >>= \case
+		Right d -> showSizeKeys d
+		Left n -> pure n
 
 known_annex_files :: Bool -> Stat
 known_annex_files isworktree = 
@@ -524,20 +528,22 @@ cachedPresentData = do
 			put s { presentData = Just v }
 			return v
 
-cachedRemoteData :: UUID -> StatState KeyInfo
+cachedRemoteData :: UUID -> StatState (Either String KeyInfo)
 cachedRemoteData u = do
 	s <- get
 	case M.lookup u (repoData s) of
-		Just v -> return v
+		Just v -> return (Right v)
 		Nothing -> do
 			let combinedata d uk = finishCheck uk >>= \case
 				Nothing -> return d
 				Just k -> return $ addKey k d
-			(ks, cleanup) <- lift $ loggedKeysFor' u
-			v <- lift $ foldM combinedata emptyKeyInfo ks
-			liftIO $ void cleanup
-			put s { repoData = M.insert u v (repoData s) }
-			return v
+			lift (loggedKeysFor' u) >>= \case
+				Just (ks, cleanup) -> do
+					v <- lift $ foldM combinedata emptyKeyInfo ks
+					liftIO $ void cleanup
+					put s { repoData = M.insert u v (repoData s) }
+					return (Right v)
+				Nothing -> return (Left "not available in this read-only repository with unmerged git-annex branches")
 
 cachedReferencedData :: StatState KeyInfo
 cachedReferencedData = do