Redo refactor gbp calls
This MR amends the previously merged !7 (merged) to do some things differently.
First of all, revert to the usage of TemporaryDirectory; to make sure the same code can still be used in both places without duplication, move the gbp import code into a separate function which can still be called within the TemporaryDirectory context.
On the other hand, creating extra branches doesn’t depend on that context, so can be safely moved outside of it.
Setting GBP_CONF_FILES is necessary for all gbp calls, so it is also
taken out from that function and directly into do_import instead.