Include metadata when asked to restore individual non-directory paths.

Adjust bup restore to handle cases like this:

  bup restore -C tmp backup/latest/somewhere/not-a-directory

Previously bup wouldn't restore the metadata for not-a-directory; now
it should.

Add wvtest-bup.sh which includes wvmktempdir().  This function creates
all temporary directories under t/tmp, making cleanup easier.

Add t/test-restore-single-file.sh

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