leanproject get-cache: get outdated

Stream: general

[Mario Carneiro]

Currently, if the latest commit of a PR is in progress, leanproject up becomes somewhat self-defeating, because it always pulls to get the latest version even if that commit doesn't have a cache. Would it be possible to modify the behavior so that it gets the last version that has a cache? It should also set the remaining files as dirty so that lean knows to compile them, if necessary (I think this is automatic).

[Bryan Gin-ge Chen]

This was also suggested here. (I haven't had time to work on this.)