import-orig: new function for filtering/re-packing sources

import-orig: new function for filtering/re-packing sources

Introduces a new function prepare_sources() that prepares upstream
sources for importing into upstream branch and pristine-tar. That
includes unpacking, filtering and re-packing sources. What somewhat
complicates the logic is that it tries to avoid excess unpacking/packing
of the sources.

Also fixes the unpacking / filtering / repacking logic which was broken
with some parameter combinations.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Conflicts:
	tests/test_import_orig.py
	tests/testutils.py