A git queue is used when doing things that update .git/index.
If two git-annex processes are both building up a git queue and happen
to flush at the same time, one will fail due to git's locking of the index
file.

This doesn't affect multiple threads with --jobs; while threads have
individual git queues, inter-thread locking allows only one to flush
at a time. That locking is handled in `Annex.Queue.flush'` using a
semaphore.

Seems that it would be better to use a lock file. --[[Joey]]

> [[done]] --[[Joey]]
