The GitPython module depends in the git command.

Installing git pulls quite some dependencies: eg. in a Debian Buster
system it increases the used space in ~100MB.

We may want to use some of the ci-fairy commands in systems with
constrained space. Since git is used only in some of the commands,
let's import only when it's really needed.

v2:
  - Remove the get_git() helper and patch the git module at
    sys.modules in the tests (Peter).

Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>