Support triggering gitlab-ci on backport branches

We'll need to support testing backport branches. Not sure of the best way to handle this right now. Manual triggers might be the way to do this. However, this is missing the --merged support without some magic behind-the-scenes reference juggling.

Only urgent once a project starts having gitlab-ci on a backport target branch (VTK 9.0 is likely to be the first).

Cc: @brad.king

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