[New Command Request] Add an option to pull from origin only, ignoring upstream

[physcrowley]

Is your feature request related to a problem? Please describe.
see #77

This is to avoid the error message:

failed to run gh: can't sync because there are diverging changes; use `--force` to overwrite the destination branch. error: exit status 1

when student repos aren't in sync with upstream (and may not need to be).

Describe the solution you'd like
As well as the current gh classroom pull student-repos command, a version including the -o flag so the execution is equivalent to git pull origin main instead of the default behaviour (which considers the entire fork chain, i.e. both origin and upstream).

Either:

gh classroom pull -o student-repos

or

gh classroom pull student-repos -o

Describe alternatives you've considered
I've both reverted to my pre-gh classroom scripts and to iterating with a simple shell script through the clones in the x-submissions directory to run git pull origin main.

Additional context
Add any other context or screenshots about the feature request here.