fsck: Hide error when probing par2 capabilities

fsck: Hide error when probing par2 capabilities

bup-fsck checks for the availability of par2's -t1 option by executing
it (for the reason see dc10c9172e1197c77c246ea75aae63388dcd641e).  If
the option is unavailable, par2 returns an error ("Invalid option
specified: -t1") which is not muted by the -qq option.

To hide this harmless par2 error during bup-fsck invocations the
subprocess call needs to capture stderr.

Signed-off-by: Gernot Schulz <post@gernot-schulz.com>
[rlb@defaultvalue.org: expand abbreviated hash to full hash above]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>