feat: create git bundle snapshots periodically

[alecthomas]

Interestingly this doesn't seem to result in significant gains. It's also quite costly in terms of storage, so it's disabled by default. That said, it likely does reduce CPU utilisation on the cache server, as it can just stream the bundle back and not have to compute it on the fly.

I was planning to automatically inject bundle-uri commands in the prototocol response, but I think given the minimal improvements I'll defer that.

Completely uncached clone:

~/dev/cachew $ rm -rf linux-source ; time git clone http://127.0.0.1:8080/git/github.com/torvalds/linux linux-source
Cloning into 'linux-source'...
remote: Enumerating objects: 11276289, done.
remote: Counting objects: 100% (98/98), done.
remote: Compressing objects: 100% (56/56), done.
remote: Total 11276289 (delta 68), reused 49 (delta 42), pack-reused 11276191 (from 1)
Receiving objects: 100% (11276289/11276289), 5.65 GiB | 7.55 MiB/s, done.
Resolving deltas: 100% (9153747/9153747), done.
Updating files: 100% (92194/92194), done.
git clone http://127.0.0.1:8080/git/github.com/torvalds/linux linux-source  486.00s user 112.07s system 67% cpu 14:50.60 total

Using the mirror on the cache server:

~/dev/cachew $ rm -rf linux-source ; time git clone http://127.0.0.1:8080/git/github.com/torvalds/linux linux-source
Cloning into 'linux-source'...
remote: Enumerating objects: 11084152, done.
remote: Counting objects: 100% (11084152/11084152), done.
remote: Compressing objects: 100% (2034375/2034375), done.
remote: Total 11084152 (delta 8997999), reused 11084152 (delta 8997999), pack-reused 0 (from 0)
Receiving objects: 100% (11084152/11084152), 5.58 GiB | 101.10 MiB/s, done.
Resolving deltas: 100% (8997999/8997999), done.
Updating files: 100% (92194/92194), done.
git clone http://127.0.0.1:8080/git/github.com/torvalds/linux linux-source  410.10s user 86.08s system 176% cpu 4:40.95 total

Using the mirror on the cache server AND the bundle:

~/dev/cachew $ rm -rf linux-source ; time git clone --bundle-uri http://127.0.0.1:8080/git/github.com/torvalds/linux.git/bundle http://127.0.0.1:8080/git/github.com/torvalds/linux.git linux-source
Cloning into 'linux-source'...
remote: Enumerating objects: 908, done.
remote: Counting objects: 100% (908/908), done.
remote: Compressing objects: 100% (908/908), done.
remote: Total 908 (delta 0), reused 908 (delta 0), pack-reused 0 (from 0)
Receiving objects: 100% (908/908), 414.26 KiB | 17.26 MiB/s, done.
Checking connectivity: 908, done.
Updating files: 100% (92194/92194), done.
git clone --bundle-uri   linux-source  358.40s user 81.89s system 197% cpu 3:42.47 total