NOBUG: Suggest to remove existing gitmirror directory

If directory gitmirror already exists, the installation process keep outputting errors:

fatal: destination path 'gitmirror' already exists and is not an empty directory.
fatal: remote public already exists.
fatal: A branch named 'MOODLE_26_STABLE' already exists.
fatal: A branch named 'MOODLE_25_STABLE' already exists.
fatal: A branch named 'MOODLE_24_STABLE' already exists.

This suggests to remove the existing directory before continue with installation