enable LambdaCase and convert around 10% of places that could use it

Needs ghc 7.6.1, so minimum base version increased slightly. All builds
are well above this version of ghc, and debian oldstable is as well.

Code that could use lambdacase can be found by running:
git grep -B 1 'case ' | less
and searching in less for "<-"

This commit was sponsored by andrea rota.

Annex/Content.hs

@@ -149,30 +149,25 @@ inAnnexSafe key = inAnnex' (fromMaybe True) (Just False) go key
 			( checkOr is_unlocked lockfile
 			, return is_missing
 			)
-	checkOr d lockfile = do
-		v <- checkLocked lockfile
-		return $ case v of
-			Nothing -> d
-			Just True -> is_locked
-			Just False -> is_unlocked
+	checkOr d lockfile = checkLocked lockfile >>= return . \case
+		Nothing -> d
+		Just True -> is_locked
+		Just False -> is_unlocked
 #else
 	checkindirect f = liftIO $ ifM (doesFileExist f)
-		( do
-			v <- lockShared f
-			case v of
-				Nothing -> return is_locked
-				Just lockhandle -> do
-					dropLock lockhandle
-					return is_unlocked
+		( lockShared f >>= \case
+			Nothing -> return is_locked
+			Just lockhandle -> do
+				dropLock lockhandle
+				return is_unlocked
 		, return is_missing
 		)
 	{- In Windows, see if we can take a shared lock. If so, 
 	 - remove the lock file to clean up after ourselves. -}
 	checkdirect contentfile lockfile =
 		ifM (liftIO $ doesFileExist contentfile)
-			( modifyContent lockfile $ liftIO $ do
-				v <- lockShared lockfile
-				case v of
+			( modifyContent lockfile $ liftIO $
+				lockShared >>= \case
 					Nothing -> return is_locked
 					Just lockhandle -> do
 						dropLock lockhandle
@@ -428,8 +423,7 @@ checkDiskSpace' need destdir key alreadythere samefilesystem = ifM (Annex.getSta
 		inprogress <- if samefilesystem
 			then sizeOfDownloadsInProgress (/= key)
 			else pure 0
-		free <- liftIO . getDiskFree =<< dir
-		case free of
+		dir >>= liftIO . getDiskFree >>= \case
 			Just have -> do
 				reserve <- annexDiskReserve <$> Annex.getGitConfig
 				let delta = need + reserve - have - alreadythere + inprogress
@@ -581,9 +575,8 @@ data FromTo = From | To
  -}
 linkAnnex :: FromTo -> Key -> FilePath -> Maybe InodeCache -> FilePath -> Maybe FileMode -> Annex LinkAnnexResult
 linkAnnex _ _ _ Nothing _ _ = return LinkAnnexFailed
-linkAnnex fromto key src (Just srcic) dest destmode = do
-	mdestic <- withTSDelta (liftIO . genInodeCache dest)
-	case mdestic of
+linkAnnex fromto key src (Just srcic) dest destmode =
+	withTSDelta (liftIO . genInodeCache dest) >>= \case
 		Just destic -> do
 			cs <- Database.Keys.getInodeCaches key
 			if null cs
@@ -602,17 +595,15 @@ linkAnnex fromto key src (Just srcic) dest destmode = do
 	failed = do
 		Database.Keys.addInodeCaches key [srcic]
 		return LinkAnnexFailed
-	checksrcunchanged = do
-		mcache <- withTSDelta (liftIO . genInodeCache src)
-		case mcache of
-			Just srcic' | compareStrong srcic srcic' -> do
-				destic <- withTSDelta (liftIO . genInodeCache dest)
-				Database.Keys.addInodeCaches key $
-					catMaybes [destic, Just srcic]
-				return LinkAnnexOk
-			_ -> do
-				liftIO $ nukeFile dest
-				failed
+	checksrcunchanged = withTSDelta (liftIO . genInodeCache src) >>= \case
+		Just srcic' | compareStrong srcic srcic' -> do
+			destic <- withTSDelta (liftIO . genInodeCache dest)
+			Database.Keys.addInodeCaches key $
+				catMaybes [destic, Just srcic]
+			return LinkAnnexOk
+		_ -> do
+			liftIO $ nukeFile dest
+			failed
 
 {- Hard links or copies src to dest, which must not already exists.
  -