If a string is passed in BaseRepo.get_walker we turn it into a one item list instead of iterating over the string itself

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>
Add test case for using a single string instead of an iterable as a valid argument for BaseRepo.get_walker

Signed-off-by: Jelmer Vernooij <jelmer@samba.org>