feat: implement git mirror

[alecthomas]

This is the first step towards the full Git caching strategy outlined in docs/git-strategy-research.md. Note that it does not yet use cachew's caching infrastructure, it solely relies on a local on-disk git mirror.

It already shows very promising results:

~/dev/cachew $ grep -A1 'url.*127.0.0' ~/.gitconfig
[url "http://127.0.0.1:8080/git/github.com/"]
  insteadOf = https://github.com/

~/dev/cachew $ rm -rf ftl ; time git clone https://github.com/block/ftl.git
Cloning into 'ftl'...
remote: Enumerating objects: 101725, done.
remote: Counting objects: 100% (1367/1367), done.
remote: Compressing objects: 100% (595/595), done.
remote: Total 101725 (delta 1106), reused 776 (delta 772), pack-reused 100358 (from 4)
Receiving objects: 100% (101725/101725), 112.49 MiB | 17.08 MiB/s, done.
Resolving deltas: 100% (61431/61431), done.
git clone https://github.com/block/ftl.git  3.42s user 0.80s system 48% cpu 8.686 total

~/dev/cachew $ rm -rf ftl ; time git clone https://github.com/block/ftl.git
Cloning into 'ftl'...
remote: Enumerating objects: 101725, done.
remote: Counting objects: 100% (101725/101725), done.
remote: Compressing objects: 100% (26203/26203), done.
remote: Total 101725 (delta 61478), reused 101451 (delta 61240), pack-reused 0 (from 0)
Receiving objects: 100% (101725/101725), 112.36 MiB | 106.24 MiB/s, done.
Resolving deltas: 100% (61478/61478), done.
git clone https://github.com/block/ftl.git  3.13s user 0.75s system 172% cpu 2.245 total