[update-checkout] add --partial-clone

[charles-zablit]

This patch adds ways to speed up a fresh clone by doing 2 things:

1. Doing a shallow clone (--depth 1) and only cloning one branch (--single-branch) when using the --skip-history flag.
2. Fetch blobs on demand with --filter=blob:none with the --partial-clone flag.

This yields a 3x performance improvement when cloning all the repositories at desk:

./swift/utils/update-checkout --clone --clone-with-ssh --scheme main --skip-history --partial-clone
56.76s user 65.67s system 78% cpu 2:36.42 total

./swift/utils/update-checkout --clone --clone-with-ssh --scheme main 
471.02s user 184.06s system 156% cpu 6:58.65 total

[charles-zablit]

@swift-ci please smoke test

[finagolfin]

@justice-adams-apple, I see that none of these flags are used on CI, would be good to add them where possible. We tried on the Android CI, but I see it isn't being applied.

Also, the Android PR CI doesn't seem to apply pulls from other repos, which Mishal said is a simple fix to add the --github-comment flag in your CI script, would be great if you'd add that too. 😺