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/TransferSlots.hs

@@ -28,7 +28,7 @@ import qualified Remote
 import qualified Types.Remote as Remote
 import Annex.Content
 import Annex.Wanted
-import Config.Files
+import Annex.Path
 import Utility.Batch
 
 import qualified Data.Map as M 
@@ -284,7 +284,7 @@ startTransfer t = do
 		alterTransferInfo t $ \i -> i { transferPaused = False }
 		liftIO $ throwTo tid ResumeTransfer
 	start info = do
-		program <- liftIO readProgramFile
+		program <- liftIO programPath
 		batchmaker <- liftIO getBatchCommandMaker
 		inImmediateTransferSlot program batchmaker $
 			genTransfer t info