The Git checkout process attempted to create a branch for the purpose

of cloning a working area. Git doesn't provide any official, reliable

mechanism for cloning form a specific commit, so we first create the

branch on the desired commit and then clone that.
This ultimately doesn't work well, since if there are two tools that

both need a workarea from the same commit, they can end up trying to

create, clone from, and destroy the same exact branch, leading to race

conditions.
We now include a UUID along with the branch name, which should help

mitigate this issue. We also attempt to destroy the branch after we

clone even if that clone operation fails.
We also no longer prune branches when fetching, which led to another

race condition where in-progress branches could be removed prematurely.

Tested with a bunch of parallel tool runs across multiple workers with

a shared set of repository directories. No longer encountered errors

creating branches, cloning, or cleaning up branches.