.gitattributes: treat *.y4m as binary data (#1187)

diff --git a/.gitattributes b/.gitattributes
index efdba87..4fb4d21 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -1,2 +1,3 @@
 * text=auto
 *.sh text eol=lf
+*.y4m -text -diff