gitattributes don't work

I'm trying to exclude certain paths (those that contain "xmac/gen/") from 
diff output using .git/info/attributes (not .gitattributes).

According to gitattributes(5) it supports patterns from gitignore(5).

Example path that must be excluded:
src/byucc/jhdl/CSRC/xmac/gen/and2_dp_g.xmac

What I've tried but didn't work:
xmac/gen/ -diff

Following works but is not what I want:
*.xmac -diff

It seems I can only get it to work for file names but not for whole paths.
What am I doing wrong or is this a bug ?

git version 1.7.3.4