prevent git-annex log with read-only unmerged git-annex branches

It would display incomplete information, which would differ from the
information displayed with write access. So refuse to display anything.

Sponsored-by: Dartmouth College's Datalad project

Command/Log.hs

@@ -1,6 +1,6 @@
 {- git-annex command
  -
- - Copyright 2012, 2016 Joey Hess <id@joeyh.name>
+ - Copyright 2012-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -82,15 +82,16 @@ optParser desc = LogOptions
 	mkpassthru n v = [Param ("--" ++ n), Param v]
 
 seek :: LogOptions -> CommandSeek
-seek o = do
+seek o = ifM (null <$> Annex.Branch.getUnmergedRefs)
+	( do
 		m <- Remote.uuidDescriptions
 		zone <- liftIO getCurrentTimeZone
 		let outputter = mkOutputter m zone o
 		let seeker = AnnexedFileSeeker
 			{ startAction = start o outputter
 			, checkContentPresent = Nothing
-		-- the way this uses the location log would not be helped
+			-- the way this uses the location log would not be
-		-- by precaching the current value
+			-- helped by precaching the current value
 			, usesLocationLog = False
 			}
 		case (logFiles o, allOption o) of
@@ -98,6 +99,8 @@ seek o = do
 				=<< workTreeItems ww fs
 			([], True) -> commandAction (startAll o outputter)
 			(_, True) -> giveup "Cannot specify both files and --all"
+	, giveup "This repository is read-only, and there are unmerged git-annex branches, which prevents displaying location log changes. (Set annex.merge-annex-branches to false to ignore the unmerged git-annex branches.)"
+	)
   where
 	ww = WarnUnmatchLsFiles