Integrate gitlab-ci/use-two-dashes-as-separator and 2 more
list_commits: use "--" as separator
Unfortunately, GitLab still interpreted three of the four dashes as start and end of YAML syntax, so we go back to two dashes.
--
list_commits: use "----" as separator
The former "---" was still rendered as YAML syntax by GitLab in some cases.
--
create_merge_request: optimize description
Restore the "---" separator which was removed in a previous commit. The description simply must not start with this separator. If we include it only in between the blocks, all instances are rendered as a horizontal line, which makes the MR description more readable. Include a helper function for this purpose, which extracts the message body from a commit message.