qubes/backup: fix relative path calculation
qubes/backup: fix relative path calculation

os.path.relpath strip trailing '/.' from the path, but it is important
to distinguish whole-directory archive (which is tar of '.').

woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin