Implement NonLinearFlowException Handling

[dkurepa]

Context

Currently when we try to flow a build from a different branch than the one that's in the vmr/repo, we get a non linear flow exception. This exception is not handled, and always results in a failed workitem.
When we see this failed workitem, we manually resolve the issue, using the --unsafe flag in the darc vmr *flow commands.

We want to automate this process as much as we can, so when we detect that this is happening, we want the service to do the --unsafe flow us, and open a PR with clear messaging that reviewers need to take extra care when reviewing.
We also want to mention the vmr reset as an alternative if the repo owners don't care about losing some changes in the VMR.

Flow

flowchart TD
    trigger[Codeflow triggered]
    linearity[Codeflow linearity check fails]
    olderCommit[Are we trying to flow an older commit?]
    doNothing[Do nothing]
    throwError[Throw error NonLinearityException]
    prUpdaterCatch[PR Updater catches NonLinearityException]
    prInProgress[Is there an in Progress PR?]
    closePR[Close PR with a message]
    isItABackflow[Is it a backflow?]
    checkVmr[Check if we're trying to backflow contents of one branch to a different one]
    error[Error: We can't backflow contents of one branch to a different one. Please create a new PR with the correct target branch.]
    unsafeFlow[Retry the codeflow operation with unsafe flag]
    success[Was the unsafe flow succesfull?]
    error1[Error: The unsafe flow also failed. Please investigate the issue and try again.]
    anyConflicts[Are there any conflicts?]
    conflict[Open an empty PR with the resolve-conflict message, but we also add the `--unsafe` flag to it. The message also says that this will be an unsafe flow and to be careful when reviewing it]
    noConflict[Open a PR with the codeflow changes, say that it was made using the `--unsafe` flag and to be careful when reviewing it]

    trigger --> linearity
    linearity --> olderCommit
    olderCommit -->|No| throwError
    olderCommit -->|Yes| doNothing
    throwError --> prUpdaterCatch
    prUpdaterCatch --> prInProgress
    prInProgress -->|Yes| closePR
    prInProgress -->|No| isItABackflow
    closePR --> isItABackflow
    isItABackflow -->|Yes| checkVmr
    isItABackflow -->|No| unsafeFlow
    checkVmr -->|Yes| error
    checkVmr -->|No| unsafeFlow
    unsafeFlow --> success
    success --> |No| error1
    success --> |Yes| anyConflicts
    anyConflicts -->|Yes| conflict
    anyConflicts -->|No| noConflict

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Goal

• Handle NonLinearExceptions where possible
• When opening a PR have CLEAR messaging about the nature of the --unsafe flow