ci-fairy: do not import git from the top
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 that module only when really needed.
Signed-off-by: Andres Gomez agomez@igalia.com