Merge branch 'ls/clean-smudge-override-in-config'

Clean/smudge filters defined in a configuration file of lower
precedence can now be overridden to be a pass-through no-op by
setting the variable to an empty string.

* ls/clean-smudge-override-in-config:
  convert: treat an empty string for clean/smudge filters as "cat"

convert.c

@@ -434,7 +434,7 @@ static int apply_filter(const char *path, const char *src, size_t len, int fd,
 	struct async async;
 	struct filter_params params;
 
-	if (!cmd)
+	if (!cmd || !*cmd)
 		return 0;
 
 	if (!dst)