`-F` is only added if *use regular expressions* is not checked (or at least it should, I didn't test right now), so it should be preserved (or adapted to work everywhere).
More generally, I agree with @elextr: we should have something configurable, but not drop current features or make them not work by default. The current code is supposed to handle *grep*, not every tool that try and replace it, in a possibly incompatible way. I myself currently use a wrapper script to use `git grep` out of the box on Git repos -- it's not too pretty but it works fine.