Add lesson to Unit 2 on more advanced git for individual use

[GA-MEB]

e.g.
using git diff, git stash, git checkout (for visiting previous commits), and interactive and patch modes for git add

Maybe a deeper treatment of git merge as well.

[Formerly https://github.com/ga-wdi-boston/010/issues/5, now migrated over]

[jrhorn424]

I'm not sure this is still a pain point. Anyone care to comment? @ga-wdi-boston/core

[gaand]

I think a practice or study might be appropriate. Something to make developers probe at the plethora of functionality that git provides.