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/ChangedRefs.hs

@@ -39,31 +39,26 @@ data ChangedRefsHandle = ChangedRefsHandle DirWatcherHandle (TBMChan Git.Sha)
 -- When possible, coalesce ref writes that occur closely together
 -- in time. Delay up to 0.05 seconds to get more ref writes.
 waitChangedRefs :: ChangedRefsHandle -> IO ChangedRefs
-waitChangedRefs (ChangedRefsHandle _ chan) = do
-	v <- atomically $ readTBMChan chan
-	case v of
+waitChangedRefs (ChangedRefsHandle _ chan) =
+	atomically (readTBMChan chan) >>= \case
 		Nothing -> return $ ChangedRefs []
 		Just r -> do
 			threadDelay 50000
 			rs <- atomically $ loop []
 			return $ ChangedRefs (r:rs)
   where
-	loop rs = do
-		v <- tryReadTBMChan chan
-		case v of
-			Just (Just r) -> loop (r:rs)
-			_ -> return rs
+	loop rs = tryReadTBMChan chan >>= \case
+		Just (Just r) -> loop (r:rs)
+		_ -> return rs
 
 -- | Remove any changes that might be buffered in the channel,
 -- without waiting for any new changes.
 drainChangedRefs :: ChangedRefsHandle -> IO ()
 drainChangedRefs (ChangedRefsHandle _ chan) = atomically go
   where
-	go = do
-		v <- tryReadTBMChan chan
-		case v of
-			Just (Just _) -> go
-			_ -> return ()
+	go = tryReadTBMChan chan >>= \case
+		Just (Just _) -> go
+		_ -> return ()
 
 stopWatchingChangedRefs :: ChangedRefsHandle -> IO ()
 stopWatchingChangedRefs h@(ChangedRefsHandle wh chan) = do