resolve merge conflicts in git-auto-export repositories

[pdpinch]

We're seeing occasional errors that indicate a merge conflict:

https://sentry.io/organizations/mit-office-of-digital-learning/issues/2812912811/events/e02444636044473f9d885923fea58cc7/?project=1757731

If a course author is doing multiple changes and publishes in a short period of time, it's possible for these changes to be executed on different nodes in the cluster. Each node will have it's own working directory, which can get out of sync with the upstream repository.

[pdpinch]

If the local branch is stale, I don't think there's any hope of updating it. We should just do a a git pull in this situation and then try export and git push again.

[pdpinch]

This is still happening: https://mit-office-of-digital-learning.sentry.io/issues/2812912811/?project=1757731

It threatens our ability to rely on the git auto exports as an archive of course changes. However, if hasn't affected anyone's actual work at this time.