Cribbed directly from the Element Flatpak's .gitignore file at
https://github.com/flathub/im.riot.Riot/blob/master/.gitignore, simply
because Element is a pretty popular application.
Note in particular that this covers all directories that will be
created by the Flatpak tutorial's suggested `flatpak-builder` command:
https://docs.flatpak.org/en/latest/first-build.html#build-and-install
`repo` is the OSTree repository that the built Flatpak will be
"published" to for testing, `.flatpak-builder` is an internal build
directory, and `builddir` is another, user-specified build directory.
