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/LockPool/PosixOrPid.hs

@@ -49,12 +49,10 @@ tryLockExclusive m f = tryPidLock m f $ Posix.tryLockExclusive m f
 checkLocked :: LockFile -> Annex (Maybe Bool)
 checkLocked f = Posix.checkLocked f `pidLockCheck` checkpid
   where
-	checkpid pidlock = do
-		v <- Pid.checkLocked pidlock
-		case v of
-			-- Only return true when the posix lock file exists.
-			Just _ -> Posix.checkLocked f
-			Nothing -> return Nothing
+	checkpid pidlock = Pid.checkLocked pidlock >>= \case
+		-- Only return true when the posix lock file exists.
+		Just _ -> Posix.checkLocked f
+		Nothing -> return Nothing
 
 getLockStatus :: LockFile -> Annex LockStatus
 getLockStatus f = Posix.getLockStatus f