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/WebApp/Configurators/Ssh.hs

@@ -28,7 +28,7 @@ import Logs.UUID
 import Assistant.RemoteControl
 import Types.Creds
 import Assistant.CredPairCache
-import Config.Files
+import Annex.Path
 import Utility.Tmp
 import Utility.FileMode
 import Utility.ThreadScheduler
@@ -381,7 +381,7 @@ sshAuthTranscript sshinput opts input = case inputAuthMethod sshinput of
 		Just (fromMaybe "" input)
 
 	setupAskPass = do
-		program <- liftIO readProgramFile
+		program <- liftIO programPath
 		v <- getCachedCred login
 		liftIO $ case v of
 			Nothing -> go [passwordprompts 0] Nothing