A common workflow using gbp is to import-orig into an experimental
branch when upgrading a package to a new version. The upgrade can be
pushed to experimental before releasing to sid.

By default gbp imports into the master branch. Importing elsewhere
needs to be specified explicitly with the --debian-branch option.

However, if you're working in an experimental branch and run gbp
import-orig, it silently imports into the master branch.  This breaks
expectations, you might not realise the import went wrong until you try
to build and wonder why it complains and fails.

import-orig should either import into the current branch
(experimental, master or otherwise),
or else it should refuse to import if the current branch is not master
and provide a warning message (if --debian-branch is not given)