When git_common creates a new workdir, it does so by whitelisting a bunch of files, creating a temp directory, and then symlinking those files into the .git directory. It's a little bit crazy, and prone to bizarre failure modes.

It should instead use 'git worktree', the built-in solution for having multiple checkouts pointing at a single set of objects. This also makes cleanup easier, and prevents any branches created in the temp workdir from polluting the parent repo's namespace.