Git source: verify usage of BitBucket

[SaschaSchwarze0]

Related to #1116

We never actually tried to clone source from BitBucket. We should do that and especially try out their access tokens that are explained in https://support.atlassian.com/bitbucket-cloud/docs/use-oauth-on-bitbucket-cloud/#Cloning-a-repository-with-an-access-token. Today, when somebody clones using HTTPS with username/password, then we use Git's credential helper. But we don't know how this maps to the Git clone command, does it work already? To be checked, fixed if necessary, and updated in the docs if appropriate.