pullbundle: attempt to save stablerange cache after each computation
Since we do not serve the full repository, we use more stablerange starting from
a random point. These are probably not loaded yet and should be cached.
This is best effort caching, if we can't save them we won't.
{"site_id":"cf023865-0eb1-49e9-951b-29555949abf0","path":"html/"}