Set dummy merge driver merge ours .gitattributes.

People don't know that they must set the `ours` driver in order for the `merge=ours` to work, or else Git silently ignores their inexistent driver.

The confusion is even greater considering that `ours` is a valid option to `merge -s recursive -X ours` 

I have seen this on two SO questions in which the OP explicitly mentions progit:

http://stackoverflow.com/questions/5465122/gitattributes-individual-merge-strategy-for-a-file
http://stackoverflow.com/questions/14093540/tell-git-to-use-ours-merge-strategy-on-specific-files

and it catches me every time I move machines.