Shallow clones

Stream: lean4

[Julian Berman]

leanpkg does not shallow clone dependencies, which means cloning mathlib as a dependency can be slower than it needs to be. Presumably it's not worth changing that, but is it worth considering for Lake?

Newish versions of git support doing this in ways that do not require looping (in case the commit requested is not a branch HEAD -- see e.g. here though not all git servers support it yet -- if a server didn't, ISTM it may be worth considering doing a clone with depth 5 or something and then falling back to full clone if it doesn't have the right commit)