Add logic to skip the Git checkout

[Liam-Sturge]

In some circumstances we would like to skip the checkout stage, in order to run the package sync on an existing checkout.

In this case it is not necessary and in some cases, undesirable to run the checkout again so this patch adds logic to bypass it when required.

Unless the --skip-checkout option is passed, existing checkout logic remains unchanged by this patch.

CC @leandron @areusch @Mousius @driazati for review

[leandron]

Merging this as it doesn't break existing scripting and the requested change was implemented. Thanks @Liam-Sturge @driazati