Shader database: Only consider link uniqueness within a post

Sure enough this change got a hit, the download URL in both of these:

http://helixmod.blogspot.com/2012/03/sega-rally-revo-3d-fix.html
http://helixmod.blogspot.com/2012/03/call-of-duty-4-modern-warfare-fix.html

is identical - both link to:

https://s3.amazonaws.com/anisotonic/ShaderOverride.zip

Doesn't seem right to me, but at least this way I catch it.