Reporting reused packfile objects

[James Ramsay]

While investigating improvements to clone performance at GitLab we've 
been looking at how to trigger packfile reuse during clones. A challenge 
of the investigation and a future challenge of rolling out changes to 
encourage more frequent packfile reuse is knowing when packfile reuse 
kicks in and the extent of the reuse.

I notice that GitHub outputs 'pack-reused' statistics when fetching. I 
assume this is for similar reasons.

Would there be interest in including a reused packfile objects statistic 
in the output of upload-pack?

I'm happy to contribute a patch (it is quite a small change), but it 
might be more efficient to upstream the patch that GitHub appears to 
already be running in production. Peff, what do you think?

Thanks,
James