Detect and report when codeflowing non-child commits

[premun]

Context

It can happen that when branching preview branches, a repo might branch before the VMR does.

This can lead to a bad situation where the VMR preview branch contains commits from main that should not be part of it. On an example, the following two commits which had no relation in the original repo (could be conflicting):

but they are parent/child in the VMR.

Instead we want the VMR to branch before and have this situation:

Goal

We should be able to detect that this is happening by determining if the currently synced commit is a child commit of the previously synced one (on the VMR side).

We also need a mechanism to bring this to attention somehow (TBD).

[adamzip]

@premun a few questions

1. On the second graph you posted, the repo preview branches off from a commit in main (the bottom-most main commit on the repo line). Since this commit is included in the repo's preview branch, why is it ok that it's not included in the VMR's preview branch? (on your graph, the VMR branches off from that commit's parent)
2. "We should be able to detect that this is happening by determining if the currently synced commit is a child commit of the previously synced one (on the VMR side)."
   Are you saying it can be determined by checking of the currently synced preview branch commit is a descendant of some main branch previously flowed commit? (one commit being the result of flowing a preview subscription and another commit being the result of flowing the main subscription)

[premun]

1. The commit technically must be in between the last one synchronized to main and first one synchronized to preview in the VMR (because its parent and child commits are on the branch in the repo too). But there does not technically need to be a synchronization for every commit so I omitted it but I see how it caused confusion.
   This is what it would look like if we had flown each commit. The red line technically can be another flow if we had flow for every commit:
   
   The VMR commit where the red line leads to (or its content) will be synchronized into the VMR either on its own, like the red line suggest, or as part of the sync of its child commit (original graph in the issue).

2. Yes - Detect and report when codeflowing non-child commits #4973