Before, we would raise whenever the usemergedriver condition was set when merging in-memory,
which equated to "any merge with (cd, dc, or m) actions in a repo with a mergedriver script".
This was done to be as conservative as possible.
However, a better solution is to run the preprocess() script and only raise if any files are
marked to actually be driver-resolved. That way we only restart the merge if we absolutely need