Migrations: Team environments can break rebuilds

[bricelam]

Imagine the following scenario.

1. DevA scaffolds a migration adding ColumnX from Table1
2. DevB scaffolds a migration that rebuilds Table1
3. DevA commits
4. DevB pulls with no conflicts in the model snapshot and commits
5. DevC pulls and runs Apply-Migration

When Table1 is rebuilt, it will not contain ColumnX. This is because when DevB's migration was scaffolded, it did not know about ColumnX.

This will be true whether we rebuild by "binding" to the model or by scaffold an imperative table rebuild.

[rowanmiller]

We should revisit the idea of keeping a Git style tree of migrations, we may not need to try and solve this issue but we could warn when we detect multiple heads and let devs know that they need to verify things work as expected. It seems like the cases where you hit issues like this are way less now that we have the combined snapshot.

[rowanmiller]

Team Triage: We're making a lot of progress on team environments in EF Core and it seems like it only falls down in edge cases now. We should come back and look at these in the future though and see if we can do something better.

[ajcvickers]

Leaving on the backlog to consider what we can do with regards to squashing, rebasing, etc of migrations. See also #2167 and #2174.