Replace SHAs with git refs in .lib files

[cmonr]

This is related to PR #565, but not dependent on it.

This is an enhancement that:

1. should help clarify exactly what versions/releases all .lib repos are at, and
2. allows for shallow clones in Properly performs shallow clone when depth is used #565 to function, since the SHA must be tied to a git ref.

[theotherjimmy]

You're referring to tags, not branches, correct?

[cmonr]

Actually, branches could be used as well (branch heads).

If the reference is listed under git ls-remote, then it can be directly fetched from the remote, either from the ref name or the SHA.

But I would think that release tags would/should be the main use case.

[theotherjimmy]

@cmonr We have a policy of not allowing branches in .lib and .bld files, as they can move. We have defined library references to be fixed, and updates require explicit user requests. As such, I can't accept a PR that automatically swaps a SHA ref; slow clone, can't change meaning; to a branch ref; fast clone, can change meaning.