vcs: Fix empty `creatordate` for tags in `RefDescriptions`

Created by: efritz

This PR refactors the RefDescriptions method pulled from the codeintel gitserver client and into the shared vcs package. Tags are pretty weird in for-each-ref, in which we need to deref the objectname and creatordate fields to get the values of the underlying commit (rather than the time of, e.g., the tag operation).

This PR does a small refactor that solves some other possible bugs. We now use \x00 as a separator instead of : (which now seems error-prone, but not a critical issue). See refs.go, which already uses both of these features as an example.

cc @bobheadxi This could be a refactoring/vetting step for @sourcegraph/dev-experience that could have a high impact.