Windows: Improve handling of shebang in external special remote program, searching for the program in the PATH.

findShellCommand needs a full path to a file in order to check it for a
shebang on Windows. It was being run with only the base name of the external
special remote program, which would only work when it was in the current
directory.

This is why users in
https://github.com/DanielDent/git-annex-remote-rclone/pull/10 and elsewhere
were complaining that the previous improvements to git-annex didn't make
git-remote-rclone work on Windows.

Also, reworked checkearlytermination, which while it worked, seemed
to rely on a race condition. And, improved its error messages.

This commit was sponsored by Shane-o on Patreon.

Remote/External.hs

@@ -375,7 +375,8 @@ startExternal external = do
 	return st
   where
 	start errrelayer g = liftIO $ do
-		(cmd, ps) <- findShellCommand basecmd
+		cmdpath <- searchPath basecmd
+		(cmd, ps) <- maybe (pure (basecmd, [])) findShellCommand cmdpath
 		let basep = (proc cmd (toCommand ps))
 			{ std_in = CreatePipe
 			, std_out = CreatePipe
@@ -383,9 +384,8 @@ startExternal external = do
 			}
 		p <- propgit g basep
 		(Just hin, Just hout, Just herr, ph) <- 
-			createProcess p `catchIO` runerr
+			createProcess p `catchIO` runerr cmdpath
 		stderrelay <- async $ errrelayer herr
-		checkearlytermination =<< getProcessExitCode ph
 		cv <- newTVarIO $ externalDefaultConfig external
 		pv <- newTVarIO Unprepared
 		pid <- atomically $ do
@@ -409,15 +409,11 @@ startExternal external = do
 		environ <- propGitEnv g
 		return $ p { env = Just environ }
 
-	runerr _ = giveup ("Cannot run " ++ basecmd ++ " -- Make sure it's in your PATH and is executable.")
-
-	checkearlytermination Nothing = noop
-	checkearlytermination (Just exitcode) = ifM (inPath basecmd)
-		( giveup $ unwords [ "failed to run", basecmd, "(" ++ show exitcode ++ ")" ]
-		, do
-			path <- intercalate ":" <$> getSearchPath
-			giveup $ basecmd ++ " is not installed in PATH (" ++ path ++ ")"
-		)
+	runerr (Just cmd) _ =
+		giveup $ "Cannot run " ++ cmd ++ " -- Make sure it's executable and that its dependencies are installed."
+	runerr Nothing _ = do
+		path <- intercalate ":" <$> getSearchPath
+		giveup $ "Cannot run " ++ basecmd ++ " -- It is not installed in PATH (" ++ path ++ ")"
 
 stopExternal :: External -> Annex ()
 stopExternal external = liftIO $ do