Standardize refs to git commits before generating VDB

[ashsong-nv]

With #72, we are currently overwriting the ref with the absolute git commit after VDB creation is finished. However this has a couple of issues:

1. Switching from a branch ref to a commit will result in a new VDB being generated even if they are from the same absolute commit
2. When a mutable ref like a branch changes to a new commit, we will still use the cached VDB for the older commit since the ref name is the same

To avoid this issue, we should standardize refs to the absolute git commit before creating the VDB.