My first guess is that there was some corruption in your checkout.
There shouldn't be. This system has had no panics or other unexpected restarts, and I just updated it a month ago.
The error message is clear: it says that 0ad-data/COMMIT_MSG in your version of the repository is untracked (i.e. not part of the git repository, a new file), but that the changes you pulled from the server include a file at that location, and that the new file would overwrite your unknown one. In case you were really adding this locally while it was added upstream, you'd need to either move it aside, or commit it locally and merge the changes from upstream with yours.
I haven't added any files, or changed, or moved. I'm interested in what the underlying problem is because if things like this happen to a completely untouched wip, it could happen to pkgsrc when pkgsrc is in git, and it'd be good to know what the error means and what people should do about it.
At some point a good CVS -> git cheat sheet would be good, including covering problems and errors.cjep started a document for after the conversion: https://wiki.netbsd.org/users/cjep/git4pkgsrc/ but the server details are of course not filled in yet, since we haven't switched.
This is a good start (but obviously for different reasons than the issue above).
Thanks! John