Right now I've implemented it such that id and commit and version and item are reserved columns, and any user-provided data with those columns gets renamed to commit_ and id_ and so-on - see #8.

I've changed my mind. I think this tool's columns should all have a _ prefix instead (like semi-private class properties in Python).

I'm even going to do that for the _id column, mainly so I don't have to explain a special case for id.