Clone git repos without blobs initially

[purcell]

This can dramatically reduce the size of large repos, see

https://github.com/melpa/melpa/issues/5361 https://github.com/melpa/melpa/issues/7524

• We always checkout or reset the repo before building, at which point blobs will be fetched lazily
• History of individual files still works

Presumably, over time, some sort of gc on long-lived checked-out working directories will also be desirable.