Add ftp:// protocol support for git-http-fetch

Based on Sasha Khapyorsky's patch but adjusted to the refactored
"missing target" detection code.

It might have been better if the program were called
git-url-fetch but it is too late now ;-).

Signed-off-by: Junio C Hamano <junkio@cox.net>

http-fetch.c

@@ -149,7 +149,9 @@ static int missing__target(int code, int result)
 	return	/* file:// URL -- do we ever use one??? */
 		(result == CURLE_FILE_COULDNT_READ_FILE) ||
 		/* http:// and https:// URL */
-		(code == 404 && result == CURLE_HTTP_RETURNED_ERROR)
+		(code == 404 && result == CURLE_HTTP_RETURNED_ERROR) ||
+		/* ftp:// URL */
+		(code == 550 && result == CURLE_FTP_COULDNT_RETR_FILE)
 		;
 }