Make blame accept absolute paths

Blame did not always use prefix_path.

Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-blame.c

@@ -1894,9 +1894,7 @@ static unsigned parse_score(const char *arg)
 
 static const char *add_prefix(const char *prefix, const char *path)
 {
-	if (!prefix || !prefix[0])
-		return path;
-	return prefix_path(prefix, strlen(prefix), path);
+	return prefix_path(prefix, prefix ? strlen(prefix) : 0, path);
 }
 
 /*