Make convert_to_parquet CLI command create script branch
Make convert_to_parquet CLI command create a "script" branch and keep the script file on it.

This PR proposes the simplest UX approach: whenever `--revision` is not explicitly passed (i.e., when the script is in the main branch), try to create a "script" branch from the "main" branch; if the "script" branch exists already, then do nothing.

Follow-up of:
- #6795

Close #6808.

CC: @severo 