Add a quick action for /rebase

Merged Stan Hu requested to merge sh-quick-action-rebase into master

This will make it easier for people to bring their merge requests up-to-date.

Since /rebase and /merge actions are not atomic or serialized, we may run into confusing race conditions if these are applied together. For now, we just ignore the /merge and proceed with the rebase.

Relates to #29406

Edited by Stan Hu

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