Fix drecurse relative --excludes, quash duplicates, and add tests.

Fix drecurse relative --excludes, quash duplicates, and add tests.

Previously "bup drecurse --exclude bar foo" wouldn't work because bar
would be expanded to an absolute path, but drecurse would traverse
(and attempt to match against) relative paths.

Have parse_excludes() remove duplicates and sort its result.

Add some initial drecurse tests.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>