Git diff and merge with p4merge (OSX)

.gitconfig

[merge]
  keepBackup = false
	tool = p4merge
[mergetool "p4merge"]
	cmd = /Applications/p4merge.app/Contents/Resources/launchp4merge "\"$PWD/$BASE\"" "\"$PWD/$REMOTE\"" "\"$PWD/$LOCAL\"" "\"$PWD/$MERGED\""
	keepTemporaries = false
	trustExitCode = false
	keepBackup = false
[diff]
    tool = p4merge
[difftool "p4merge"]
	cmd = /Applications/p4merge.app/Contents/Resources/launchp4merge "\"$REMOTE\"" "\"$LOCAL\""

there is also a blog post http://naleid.com/blog/2013/10/29/how-to-use-p4merge-as-a-3-way-merge-tool-with-git-and-tower-dot-app