storagemgr: Convert `InitialValues` to use `git.Reference`
The InitialValues
in RecordInitialReferenceValues
uses git.ObjectID
to denote the initial value for a reference. This currently works since we only deal with regular references. We want to move to support symbolic references too, since this will be used in the context of reftables, where updating the default branch ('HEAD' reference) is treated similar to a regular ref update.
Part of #5867 (closed)