Use PHPStorm as your git diff tool

.gitconfig

# Run `git difftool <directory/file>...` or `git mergetool <directory/file>...`
[merge]
	tool = phpstorm
[diff]
	tool = phpstorm
[difftool]
	prompt = false
[difftool "phpstorm"]
	cmd = /Applications/PhpStorm\\ EAP.app/Contents/MacOS/phpstorm diff $(cd $(dirname "$LOCAL") && pwd)/$(basename "$LOCAL") $(cd $(dirname "$REMOTE") && pwd)/$(basename "$REMOTE")
	trustExitCode = true
[mergetool "phpstorm"]
	cmd = /Applications/PhpStorm\\ EAP.app/Contents/MacOS/phpstorm merge $(cd $(dirname "$LOCAL") && pwd)/$(basename "$LOCAL") $(cd $(dirname "$REMOTE") && pwd)/$(basename "$REMOTE") $(cd $(dirname "$BASE") && pwd)/$(basename "$BASE") $(cd $(dirname "$MERGED") && pwd)/$(basename "$MERGED")
	trustExitCode = true

Add this to your global .gitconfig file.

See https://gist.github.com/jvandyke/4355099#comment-1437345 for a simpler version you can use on new versions of PHPStorm and WebStorm.