disable precaching git-annex branch when there are unmerged branches in a read-only repo

The way precaching works, it can't merge in information from those
branches efficiently, so just disable it and fall back to
Annex.Branch.get in order to get the correct information.

Sponsored-by: Dartmouth College's Datalad project

Annex/Branch.hs

@@ -20,6 +20,7 @@ module Annex.Branch (
 	updateTo,
 	get,
 	getHistorical,
+	getUnmergedRefs,
 	RegardingUUID(..),
 	change,
 	maybeChange,
@@ -281,6 +282,12 @@ get file = do
 		bs <- forM refs $ \ref -> getRef ref file
 		return (l <> mconcat bs)
 
+{- When the git-annex branch is unable to be updated due to permissions,
+ - and there are other git-annex branches that have not been merged into
+ - it, this gets the refs of those branches. -}
+getUnmergedRefs :: Annex [Git.Ref]
+getUnmergedRefs = unmergedRefs <$> update
+
 {- Used to cache the value of a file, which has been read from the branch
  - using some optimised method. The journal has to be checked, in case
  - it has a newer version of the file that has not reached the branch yet.

CmdLine/Seek.hs

@@ -361,13 +361,14 @@ seekFilteredKeys seeker listfs = do
 				(process mi ofeeder mdfeeder mdcloser False l)
 			mdprocessertid <- liftIO . async =<< forkState
 				(mdprocess mi mdreader ofeeder ocloser)
-			if usesLocationLog seeker || matcherNeedsLocationLog mi
-				then catObjectStream g $ \lfeeder lcloser lreader -> do
+			ifM (precachell mi)
+				( catObjectStream g $ \lfeeder lcloser lreader -> do
 					precachertid <- liftIO . async =<< forkState
 						(precacher mi config oreader lfeeder lcloser)
 					precachefinisher mi lreader checktimelimit
 					join (liftIO (wait precachertid))
-				else finisher mi oreader checktimelimit
+				, finisher mi oreader checktimelimit
+				)
 			join (liftIO (wait mdprocessertid))
 			join (liftIO (wait processertid))
 	liftIO $ void cleanup
@@ -467,6 +468,16 @@ seekFilteredKeys seeker listfs = do
 		Just _ -> mdprocess mi mdreader ofeeder ocloser
 		Nothing -> liftIO $ void ocloser
 
+	-- Precache location logs if it will speed things up.
+	--
+	-- When there are git-annex branches that are not able to be
+	-- merged, the precaching is disabled, since it only looks at the
+	-- git-annex branch and not at those.
+	precachell mi
+		| usesLocationLog seeker || matcherNeedsLocationLog mi =
+			null <$> Annex.Branch.getUnmergedRefs
+		| otherwise = pure False
+
 seekHelper :: (a -> RawFilePath) -> WarnUnmatchWhen -> ([LsFiles.Options] -> [RawFilePath] -> Git.Repo -> IO ([a], IO Bool)) -> WorkTreeItems -> Annex ([(SeekInput, a)], IO Bool)
 seekHelper c ww a (WorkTreeItems l) = do
 	os <- seekOptions ww