use programPath consistently, not readProgramFile

Improve git-annex's ability to find the path to its program, especially
when it needs to run itself in another repo to upgrade it.

Some parts of the code used readProgramFile, probably because I forgot that
programPath exists.

I noticed this when a git-annex auto-upgrade failed because it was running
git-annex upgrade --autoonly, but the code to run git-annex used
readProgramFile, which happened to point to an older build of git-annex.

Command/Sync.hs

@@ -46,7 +46,7 @@ import Config
 import Config.GitConfig
 import Annex.SpecialRemote.Config
 import Config.DynamicConfig
-import Config.Files
+import Annex.Path
 import Annex.Wanted
 import Annex.Content
 import Command.Get (getKey')
@@ -509,7 +509,7 @@ pushRemote o remote (Just branch, _) = do
 		Nothing -> return True
 		Just wt -> ifM needemulation
 			( liftIO $ do
-				p <- readProgramFile
+				p <- programPath
 				boolSystem' p [Param "post-receive"]
 					(\cp -> cp { cwd = Just (fromRawFilePath wt) })
 			, return True