add new parameter "current git branch name" to be used with external diff tool to see branch name when making merge

When comparing changes or doing merge between two branches, it's important to see the name of the branch on left / right panels. For external diff tools there are existing  lefTitle / rightTitle parameters that can be passed to an external diff tool,

but they show HEAD and Index when comparing local changes. Instead of that I would like to see current git branch name.