Automatic upstream sync may overwrite manually applied changes applied during sync

[dagood]

There's a known race condition in the PR:

• sync: skip a push that would overwrite another's change go-infra#175
• For Auto-sync script shouldn't overwrite ongoing fixup work #68

There's a ~1 minute window (observed in a recent run) where if a dev pushes a commit to the sync PR, it may still be overwritten by the bot.

For now, it seems like the value is very low:

• The window is small.
• If it does happen, the dev can simply push again (they are likely still set up to trivially run a force push).
• Not much CI-running time is lost.
  • Losing CI results has been the larger sting for an overwrite. The commit itself was always recoverable.

The amount of investigation needed to implement a fix is probably not all that high, but the value is very low, so I'm filing this just to keep track of the known issue and potentially consider fixing it later.