unarchive: do not assume tar supports -C

Fixes #7777
(But don't rewrite the tar invocation in is_unarchived(), since a
tar that supports "--diff" certainly supports "-C" as well).
Merge branch 'unarchive-fix-7777-simple' of https://github.com/toddmowen/ansible into toddmowen-unarchive-fix-7777-simple
Merge branch 'toddmowen-unarchive-fix-7777-simple' into devel