bup-index.md: remove misleading --exclude expansion comment.

This comment was originally added in reference to some confusion over
globbing, but the text was misleading because bup does apply
realpath() to all of the --exclude paths, arguably a form of
expansion.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
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>
Add --exclude-rx and --exclude-rx-from support to "bup drecurse".

Thanks to nicolas.pouillard@gmail.com for the reminder.

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