bup drecurse/index: allow non-directory --exclude and --exclude-from paths.

Previously bup only allowed directory exclusions.

Signed-off-by: Jakob Matthes <jakob.matthes@gmail.com>
Reviewed-by: Gabriel Filion <lelutin@gmail.com>
[rlb@defaultvalue.org: change test file from g to j to avoid conflict
 with subsequent "exclude-from" test; add WVPASS to mkdir so problem
 is evident; edit commit message.]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Add "set -eo pipefail" to "exclude" and "exclude-from" tests.

Don't allow non-tests to silently fail (i.e. mkdir, touch, echo, rm,
etc.).  Eventually all test code should have similar behavior.

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