bots: Add make-checkout option to checkout a different cockpit bots/ ref
bots: Add make-checkout option to checkout a different cockpit bots/ ref

For testing external projects for changes to cockpit's bots/ directory,
we need to be able to check out the project plus Cockpit's bots/ from a
ref other than master (i. e. usually a pull request, like
pull/1234/head). Add a new `--bots-ref` option to it which does that.
When not given, bots/ will continue to be checked out from
`origin/master`.

Closes #9605

martinpitt/cockpit,mareklibra/cockpit,andreasn/cockpit,garrett/cockpit,martinpitt/cockpit,garrett/cockpit,cockpit-project/cockpit,garrett/cockpit,andreasn/cockpit,cockpit-project/cockpit,deryni/cockpit,deryni/cockpit,mareklibra/cockpit,deryni/cockpit,martinpitt/cockpit,cockpituous/cockpit,cockpituous/cockpit,deryni/cockpit,mareklibra/cockpit,mvollmer/cockpit,martinpitt/cockpit,andreasn/cockpit,mareklibra/cockpit,mvollmer/cockpit,deryni/cockpit,mareklibra/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,mareklibra/cockpit,moraleslazaro/cockpit,deryni/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,andreasn/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,mareklibra/cockpit,garrett/cockpit,andreasn/cockpit,andreasn/cockpit,cockpituous/cockpit,cockpit-project/cockpit,andreasn/cockpit,martinpitt/cockpit,cockpituous/cockpit,deryni/cockpit,garrett/cockpit