improve programPath

Fixes a failure mode where git-annex sync would try to run git-annex and
complain that it failed to find it in ~/.config/git-annex/program or PATH,
when there was a git-annex in /usr/bin/, but the original one was run
from elsewhere (eg, ~/bin) and happened not to be present any longer.

Now, it will fall back to using git-annex from PATH in such a case.
Which might fail due to some version incompatability, but still better
than a misleading error message.

Also made readProgramFile only read the file, not look for git-annex in
PATH as a fallback. That fallback may have confused Assistant.Upgrade,
which really wants the value from the file.

Annex/Path.hs

@@ -32,24 +32,14 @@ programPath = go =<< getEnv "GIT_ANNEX_PROGRAMPATH"
 		exe <- getExecutablePath
 		p <- if isAbsolute exe
 			then return exe
-			else readProgramFile
+			else fromMaybe exe <$> readProgramFile
 		maybe cannotFindProgram return =<< searchPath p
 
 {- Returns the path for git-annex that is recorded in the programFile. -}
-readProgramFile :: IO FilePath
+readProgramFile :: IO (Maybe FilePath)
 readProgramFile = do
 	programfile <- programFile
-	p <- catchDefaultIO cmd $ 
-		fromMaybe cmd . headMaybe . lines <$> readFile programfile
-	ifM (inPath p)
-		( return p
-		, ifM (inPath cmd)
-			( return cmd
-			, cannotFindProgram
-			)
-		)
-  where
-	cmd = "git-annex"
+	headMaybe . lines <$> readFile programfile
 
 cannotFindProgram :: IO a
 cannotFindProgram = do