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.
After resolving the merge with DiffMerge, does it save the file? I assume you still have to add it to git and commit the resolution? Probably should add a final step reminding people they still need to do this, and refer them back to the original git lesson.