Add squash quick action for merge requests

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Issue

Projects have a default shallow clone depth of 20 commits. For merge requests with more than 20 commits, danger-review-local on GitLab projects fails with:

fatal: not a valid branch point: '3ddffaacb3735622fc85e7efc0ec1daa6be27b57'
fatal: couldn't find remote ref refs/heads/danger_base

The current implementation makes it impossible for contributor who only uses the Web IDE from making the pipeline pass after 20 commits.

• contributor may not want to clone the GitLab repo to their local computers due to the large size (> 1 GB)
• contributor may not want to use GitPod because
  • of the long loading time (> 20 minutes on a fresh start)
  • it blocks registration from certain email address domains
  • it has a free tier limit of 10 hours (unverified) or 50 hours (verified) per month on standard class instances

Proposal

Add a /squash quick action that when called with:

• /squash: squashes all commits on merge request branch, and keep existing commit message
  • solves the danger-review-local issue
• /squash "commit message": squashes all commits on merge request branch, and sets commit message (which may be multiline)
  • this allows adding/updating a CHANGELOG entry

Implementation guide

• may be similar to the /rebase quick action
• extract message after quick action