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.hs

@@ -104,7 +104,7 @@ startDaemon assistant foreground startdelay cannotrun listenhost startbrowser =
 			( liftIO $ withFile devNull WriteMode $ \nullh -> do
 				loghandle <- openLog logfile
 				e <- getEnvironment
-				cmd <- readProgramFile
+				cmd <- programPath
 				ps <- getArgs
 				(_, _, _, pid) <- createProcess (proc cmd ps)
 					{ env = Just (addEntry flag "1" e)