Dependencies declared with non-master git branches fail with "no reference found"

[mitchmindtree]

I'm attempting to test #978 and noticed I seem unable to fetch any reference other than master 😵‍💫.

We must be missing a fetch operation for the particular branch, or perhaps an argument to the clone operation, or something along these lines.

I'll address this as a part of #978.

[mitchmindtree]

Currently we clone the temp repository, but I think the better approach might be to simply do an init and then immediately fetch with only the required refspecs. It looks like this is the approach cargo is taking.