Adding git graphical diff/merge tool lesson

Added a lesson for integrating _DiffMerge_ into Git's `difftool` and `diffmerge` commands. 
We've attempted to follow the sample repos/examples created in the previous lessons.
Looks like they don't have a specific set of instructions for git bash, which is what windows users usually use in our workshops (at least the ones I've taught). Will one of these sets of instructions work for git bash? If so, you may want to link to that directly.