Autocancel plans running on PR after new push

[paymog]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story

I push a commit to a branch. This invokes atlantis plan. I realize I made a mistake and push a second commit to the same branch. This invokes atlantis plan again, but the second invcation fails with

the default workspace at path . is currently locked by another command that is running for this pull request.\nWait until the previous command is complete and try again

Describe the solution you'd like

Atlantis should detect when the lock is held by the same PR that got the new commit. If that's the case, Atlantis should cancel the prior plan and start a new plan on the latest commit.

Describe the drawbacks of your solution

More complexity in the codebase.

Describe alternatives you've considered

Wait and invoke plan again once the old plan has finished.