In rustpkg, expose the ability to clone a git repository

[catamorphism]

Sub-bug of #5677

Package scripts should be able to call a librustpkg function to clone a git repository, given a URL and a branch. Right now this function just shells out to call git, but libgit2 bindings would be good (I'll make a separate bug).

[emberian]

See also: #6410

[catamorphism]

#9732 will close this.

[catamorphism]

#9732 fixed this. It's in rustpkg::api