src/ostree: Add --commit-only option to ostree prune

src/ostree: Add --commit-only option to ostree prune

Recently we have noticed exceedingly long execution times
for multiple invocations of ostree prune. This is a result of
calculating full reachability on each invocation.

The --commit-only flag provides an alternative strategy. It will only
traverse and delete commit objects to avoid the more expensive
reachability calculations. This allows us to chain multiple --commit-only
commands cheaply, and then follow with a more expensive ostree prune
invocation at the end to clean up orphaned meta and content objects.