fractal-gtk: Use git version of tree_magic
requested to merge BrainBlasted/fractal:wip/christopherdavis/fix-is_gif-crash-when-switching-rooms into master
tree_magic did not previously return an Option from from_filepath, and used unwrap() internally, occasionally causing a crash when we used is_gif().
On git master they have fixed this, so that's what we need to use until they put out a new release.