In order to import a git repo into Unity Version Control / Plastic, the recommended way is to use GitSync:
It seems something is wrong with the git export package you generated. The GitSync feature has been tested in multiple environments and it’s optimized to migrate git repos to Plastic. Please let us know if you still have any problems when using it.
Can you give some guidance on how to undo the import and remove all these branches? It says I can’t delete branches with changesets, and cannot delete changesets with existing references.
The docs about cm fast-import don’t say anything about using gitsync instead.