## Problem
We did not test some Public API calls, such as using a timestamp to
create a branch, reset_to_parent.
## Summary of changes
Tests now include some other operations: reset_to_parent, a branch
creation from any time in the past, etc.
Currently, the API calls are only exposed; the semantics are not
verified.
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Co-authored-by: Alexey Masterov <alexey.masterov@databricks.com>