Add index option "--exclude-rx-from FILE".

Signed-off-by: Thomas Klausner <tk@giga.or.at>
[rlb@defaultvalue.org: adjust commit message; change "path" to
 "pattern" in bup-index.md description; adjust bup-index.md text;
 adjust optspec text.]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Move the --exclude-rx tests from test.sh to test-save-restore-exclude-rx.sh.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Add simple test for "index --exclude-rx-from FILE".

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Add restore option "--exclude-rx-from FILE".

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