Add `--fileops` flag to `strip --reduce` to treat all file operations as boring

I was using strip --reduce to get a simplified view of my repository in order to inspect the branch and tag topology in a git graph visualization.

However, since strip --reduce keeps all commits with delete/copy/rename file operations, I still had way to many "boring" commits in between.

With strip --reduce --fileops, the repo is now boiled down even more, keeping only branch creations/merges/tags and their neighbors.

(feel free to rename the flag if you have a better idea than --fileops!)