Since 1.20.1, dpkg-source -x will fail the extraction for source
packages with files producing ioerrors. Detecting IOError while parsing
those files not being useful anymore, coverge and tests have been
removed to keep the pipelines for both buster and sid happy.

Split into two classes:
GitStorage for high level actions on the git storage (install, diff or
remove).
GitBackendDulwich that implements dulwich storage specifics.

     fixing empty tree bug, 2 things were bad, path was absolute and files listing was not working as it was supposed to

Conflicts:

	debexpo/importer/importer.py
	debexpo/lib/gitstorage.py