handle overBranchFileContents with read-only unmerged git-annex branches

This makes --all error out in that situation. Which is better than
ignoring information from the branches.

To really handle the branches right, overBranchFileContents would need
to both query all the branches and union merge file contents
(or perhaps not provide any file content), as well as diffing between
branches to find files that are only present in the unmerged branches.
And also, it would need to handle transitions..

Sponsored-by: Dartmouth College's Datalad project

Annex/Branch.hs

@@ -820,13 +820,32 @@ rememberTreeishLocked treeish graftpoint jl = do
  - The action is passed a callback that it can repeatedly call to read
  - the next file and its contents. When there are no more files, the
  - callback will return Nothing.
+ -
+ - In some cases the callback may return the same file more than once,
+ - with different content. This happens rarely, only when the journal
+ - contains additional information, and the last version of the
+ - file it returns is the most current one.
+ -
+ - In a read-only repository that has other git-annex branches that have
+ - not been merged in, returns Nothing, because it's not possible to
+ - efficiently handle that.
  -}
 overBranchFileContents
 	:: (RawFilePath -> Maybe v)
 	-> (Annex (Maybe (v, RawFilePath, Maybe L.ByteString)) -> Annex a)
-	-> Annex a
+	-> Annex (Maybe a)
 overBranchFileContents select go = do
 	st <- update
+	if not (null (unmergedRefs st))
+		then return Nothing
+		else Just <$> overBranchFileContents' select go st
+
+overBranchFileContents'
+	:: (RawFilePath -> Maybe v)
+	-> (Annex (Maybe (v, RawFilePath, Maybe L.ByteString)) -> Annex a)
+	-> BranchState
+	-> Annex a
+overBranchFileContents' select go st = do
 	g <- Annex.gitRepo
 	(l, cleanup) <- inRepo $ Git.LsTree.lsTree
 		Git.LsTree.LsTreeRecursive
@@ -836,7 +855,7 @@ overBranchFileContents select go = do
 	buf <- liftIO newEmptyMVar
 	let go' reader = go $ liftIO reader >>= \case
 		Just ((v, f), content) -> do
-			content' <- checkjournal st f content
+			content' <- checkjournal f content
 			return (Just (v, f, content'))
 		Nothing
 			| journalIgnorable st -> return Nothing
@@ -853,7 +872,7 @@ overBranchFileContents select go = do
 		`finally` liftIO (void cleanup)
   where
 	-- Check the journal, in case it did not get committed to the branch
-	checkjournal st f branchcontent
+	checkjournal f branchcontent
 		| journalIgnorable st = return branchcontent
 		| otherwise = getJournalFileStale (GetPrivate True) f >>= return . \case
 			NoJournalledContent -> branchcontent