git: don't re-clone on "wipe directory and start over"
For all version control types we just delete the source and build dir and start again: https://gitlab.gnome.org/GNOME/jhbuild/-/blob/5f8c17be0ef115587616b3b5737709016f5f9f20/jhbuild/versioncontrol/init.py#L159
But in the case of git we should just be able to do a combination for git reset/clean/checkout to achieve the same without re-cloning everything, which takes ages.
I got annoyed by this last time meson changed and I had to wipe every module because meson failed to reconfigure.