satellites: fix checkout sequence when merging requests

Created by: davvid

"git checkout -b" will fail when the target branch already exists in the satellite repository, which makes gitlab unable to process merge requests.

Change the checkout sequence to be more robust. Use "git checkout " so that an existing branch can be used if it already exists.

Use "git reset --hard" and "git clean" to ensure that the state is known before attempting the merge.

Closes #3697 (closed)