add annex-ignore-command and annex-sync-command configs

Added remote configuration settings annex-ignore-command and
annex-sync-command, which are dynamic equivilants of the annex-ignore
and annex-sync configurations.

For this I needed a new DynamicConfig infrastructure. Its implementation
should be as fast as before when there is no dynamic config, and it caches
so shell commands are only run once.

Note that annex-ignore-command exits nonzero when the remote should be ignored.
While that may seem backwards, it allows using the same command for it as
for annex-sync-command when you want to disable both.

This commit was sponsored by Trenton Cronholm on Patreon.

Config/DynamicConfig.hs

@@ -0,0 +1,44 @@
+{- dynamic configuration
+ -
+ - Copyright 2017 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Config.DynamicConfig where
+
+import Control.Concurrent.STM
+
+import Utility.SafeCommand
+
+-- | A configuration value that may only be known after performing an IO
+-- action. The IO action will only be run the first time the configuration
+-- is accessed; its result is then cached.
+data DynamicConfig a = DynamicConfig (IO a, TMVar a) | StaticConfig a
+
+mkDynamicConfig :: CommandRunner a -> Maybe String -> a -> STM (DynamicConfig a)
+mkDynamicConfig _ Nothing static = return $ StaticConfig static
+mkDynamicConfig cmdrunner (Just cmd) _ = do
+	tmvar <- newEmptyTMVar
+	return $ DynamicConfig (cmdrunner cmd, tmvar)
+
+getDynamicConfig :: DynamicConfig a -> IO a
+getDynamicConfig (StaticConfig v) = return v
+getDynamicConfig (DynamicConfig (a, tmvar)) = 
+	go =<< atomically (tryReadTMVar tmvar)
+  where
+	go Nothing = do
+		v <- a
+		atomically $ do
+			_ <- tryTakeTMVar tmvar
+			putTMVar tmvar v
+		return v
+	go (Just v) = return v
+
+type CommandRunner a = String -> IO a
+
+successfullCommandRunner :: CommandRunner Bool
+successfullCommandRunner cmd = boolSystem "sh" [Param "-c", Param cmd]
+
+unsuccessfullCommandRunner :: CommandRunner Bool
+unsuccessfullCommandRunner cmd = not <$> successfullCommandRunner cmd