Add external tool variable for current branch for file commands

I would like to be able to configure an external tool to work on a file, in the branch I am looking at it in.

For example, I would want to create a command the shows the file I am looking at in GitHub. I have the remote url already, and the path to the file, and the root of the repo to make the relative path, the only bit I'm missing is which branch I'm looking at.