Run auto GC in the background

Run auto GC in the background

When running an automatic GC on a FileRepository, when the caller
passes a NullProgressMonitor, run the GC in a background thread. Use a
thread pool of size 1 to limit the number of background threads spawned
for background gc in the same application. In the next minor release we
can make the thread pool configurable.

In some cases, the auto GC limit is lower than the true number of
unreachable loose objects, so auto GC will run after every (e.g) fetch
operation.  This leads to the appearance of poor fetch performance.
Since these GCs will never make progress (until either the objects
become referenced, or the two week timeout expires), blocking on them
simply reduces throughput.

In the event that an auto GC would make progress, it's still OK if it
runs in the background. The progress will still happen.

This matches the behavior of regular git.

Git (and now jgit) uses the lock file for gc.log to prevent simultaneous
runs of background gc. Further, it writes errors to gc.log, and won't
run background gc if that file is present and recent. If gc.log is too
old (according to the config gc.logexpiry), it will be ignored.

Change-Id: I3870cadb4a0a6763feff252e6eaef99f4aa8d0df
Signed-off-by: David Turner <dturner@twosigma.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>