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.

Upgrade.hs

@@ -13,7 +13,7 @@ import Annex.Common
 import qualified Annex
 import qualified Git
 import Config
-import Config.Files
+import Annex.Path
 import Annex.Version
 import Types.RepoVersion
 #ifndef mingw32_HOST_OS
@@ -103,7 +103,7 @@ upgrade automatic destversion = do
 	-- upgrading a git repo other than the current repo.
 	upgraderemote = do
 		rp <- fromRawFilePath <$> fromRepo Git.repoPath
-		cmd <- liftIO readProgramFile
+		cmd <- liftIO programPath
 		liftIO $ boolSystem' cmd
 			[ Param "upgrade"
 			, Param "--quiet"