sl pr submit can overwrite another PR in race condition

[alex-statsig]

Haven't had this happen before, but twice today ran into an issue where one user's sl pr submit (via ISL UI in vscode) pushed changes to someone else's branch/PR (both created via sapling). Presumably we both submitted PRs at the same time, they both picked the same PR number, and committed changes to that branch, but theirs ultimately failed when creating the PR itself (but not before pushing changes to my branch).

I'm not sure if this is a recent regression or just a result of a high volume of PRs today.