Improve Git packfile reuse

Problem to solve

Packfile reuse decreases CPU, and disc IO when cloning or fetching large amounts of data, but it's hard to know if it was used.

Furthermore, Peff points out the initial implementation is not ideal, and provided a patch that GitHub has been running for a long time.

Further details

Proposal

Upstream the patchfile provided by Peff to:

• add output
• improve patckfile reuse

Links / reference

https://public-inbox.org/git/3E56B0FD-EBE8-4057-A93A-16EBB09FBCE0@jramsay.com.au/T/#u