git step caches should act as proper reference repositories

[rtyler]

Right now the git step is doing some funny business to cache git repositories on the agent. The git2 crate which is itself based on libgit2 doesn't overtly support using a reference repository on the clone operation.

After some discussion in #libgit2, there's potentially a workaround:

13:26:00 rtyler | my understanding for cloning with a reference repository is that the reference is used primarily, and then the "upstream" repo 
                | is the alternate (to use this parlance)
13:27:55 qyliss | I'm not sure                                                                                                       
13:28:00 qyliss | but should be easy to test                                                                                          
13:30:41 qyliss | rtyler: it looks like the odb is a property of the repository                                                          
13:30:50 qyliss | see here: https://libgit2.org/libgit2/#HEAD/group/repository/git_repository_odb                                     
13:31:22 qyliss | so I think what you'd want to do is make an empty repository, set up your remote, get the odb, set up your alternate, and then
                | fetch
13:31:36 qyliss | I think in this case the alternate would be the other local repository.                                             
13:31:45 rtyler | yeah that makes sense                       

For this issue, first need to validate the experimental approach of:

• Create bare repository
• Add remote
• Get odb
• set up alternate(s)
• fetch

If that actually works, replacing the clone() function in stdlib/git should be relatively straightforward.