GitLab MR: introduce skip commit action.

[mbenson]

Hey, I just made a Pull Request!

Introduced to GitLab MR action a skip value for commitAction that can be used to:

• generate an MR from an existing branch as requested in #26589
• with commitAction auto, generate an MR omitting unchanged workspace files

:heavy_check_mark: Checklist

• [X] A changeset describing the change and affected packages. (more info)
• [X] Added or updated documentation
• [X] Tests for new functionality and regression tests for bug fixes
• [ ] Screenshots attached (for UI changes)
• [X] All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie[bot]]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage/plugin-scaffolder-backend-module-gitlab	plugins/scaffolder-backend-module-gitlab	patch	v0.6.0-next.2

[benjdlambert]

I wonder if we should be moving in #22244 direction instead?

git:repo:push and gitlab:mr:create with a branch name instead. It feels like we're halfway there to be honest with this PR.

Not advised to merging this, just feels like it's worth a discussion first.

[mbenson]

Do we want to discuss at Scaffolder SIG this week?

[github-actions[bot]]

Thank you for contributing to Backstage! The changes in this pull request will be part of the 1.32.0 release, scheduled for Tue, 15 Oct 2024.