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deploy_gitlab_ci: show instructions if merge fails
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update_submodule: fix accidental multi-commits
If integration branch already existed and had to be replaced, the new commit was accidentally pushed on top of the already existing commit instead of replacing it. Fixed this by checking out the master branch again before creating the commit.
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list_commits: replace "---" with "--"
GitLab renders "---" as a horizontal line or as a frame. We do not want this, so replace it with "--".