Have a --force option on gitpuller

[clancychilds]

For some use cases it would be nice to be able to specify on the gitpuller command line to pull down the latest version of a notebook from a repo and obliterate any local changes. e.g. a --force option

[fmaussion]

Yes! I would find it useful to have a --force option for gitpuller links as well.

I fully understand the ideas behind the Automatic Merging Behavior, but I've seen instructors being confused by why their notebooks won't update, etc.

I usually tell students to make a working copy anyway (so that they can go back to the original assignment), and I would personally use --force as default and document that behavior. Yet, again, I understand the motivations behind it.