When re-execing git-annex, use current program location, rather than ~/.config/git-annex/program, when possible.

Most of the time, there will be no discreprancy between programPath and
readProgramFile.

But, the programFile might have been written by an old version of git-annex
that is still installed, while a newer one is currently running. In this
case, we want to run the same one that's currently running.

This is especially important for things like the GIT_SSH=git-annex used for
ssh connection caching.

The only code that still uses readProgramFile directly is the upgrade code,
which needs to know where the standalone git-annex was installed, in order to
upgrade it.

Assistant/Restart.hs

@@ -19,9 +19,9 @@ import Utility.Url
 import Utility.PID
 import qualified Git.Construct
 import qualified Git.Config
-import Config.Files
 import qualified Annex
 import qualified Git
+import Annex.Path
 
 import Control.Concurrent
 #ifndef mingw32_HOST_OS
@@ -110,7 +110,7 @@ assistantListening url = catchBoolIO $ exists url' def
  -}
 startAssistant :: FilePath -> IO ()
 startAssistant repo = void $ forkIO $ do
-	program <- readProgramFile
+	program <- programPath
 	(_, _, _, pid) <- 
 		createProcess $
 			(proc program ["assistant"]) { cwd = Just repo }