Add support for gc.pruneExpire configuration

[jelmer]

Add support for the gc.pruneExpire configuration option to control when unreachable objects are pruned during garbage collection.

This option specifies the grace period before unreachable objects are removed, allowing for recent operations to complete and providing a safety buffer. The default in Git is typically "2.weeks.ago".

Configuration format:

• gc.pruneExpire = <time> - Prune unreachable objects older than specified time

Example: gc.pruneExpire = 1.week.ago