Add --rebase option for CI mirror mode
With the default of doing 'git pull upstream master' we will end up with a merge commit. That's okay but sometimes it's better to avoid it and just rebase any local changes. The user is given the choice by using the MIRROR_PULL_MODE variable.
Additionally when doing git pull --rebase we also use git push --force since we are effectively rewriting git history. Our project repository must not have protected branches.
Closes #137 (closed)