threads: introduce `git_thread_exit`

threads: introduce `git_thread_exit`

Introduce `git_thread_exit`, which will allow threads to terminate at an
arbitrary time, returning a `void *`.  On Windows, this means that we
need to store the current `git_thread` in TLS, so that we can set its
`return` value when terminating.

We cannot simply use `ExitThread`, since Win32 returns `DWORD`s from
threads; we return `void *`.