Specify branch to be used when migrating from bitbucket pipelines

[michaelfedell]

Many of our BitBucket repositories use the default branch, but there are a few which have their pipeline defined on some non-default branch (e.g. development). It would be desirable to point the gh actions-importer at this branch when running the dry-run or migrate commands. E.g.

gh actions-importer dry-run bitbucket --workspace example --output-dir dry-run --repository repo --branch development