How to I get diffmerge running as the merge tool for git

Question

I'm running osx 10.9 with git and I'm trying to get diffmerge running as my merge tool. I've installed diffmerge and can launch it from my bash shell -

$ diffmerge

however when I try and set it up as my merge tool -

$ git config --global merge.tool diffmerge
$ git config --global mergetool.diffmerge.cmd "diffmerge --merge --result=\$MERGED \$LOCAL \$BASE \$REMOTE"
$ git config --global mergetool.diffmerge.trustExitCode true

And run

$ git mergetool

I get the following output -

git config option merge.tool set to unknown tool: diffmerge
Resetting to default...

This message is displayed because 'merge.tool' is not configured.

Any idea's why this is failing? (it might be worth noting that diffmerge isn't listed as a valid tool when I run git mergetool --tool-help)