Bug 21908: Add DISTINCT biblionumber to rebuild_zebra.pl

Bug 21908: Add DISTINCT biblionumber to rebuild_zebra.pl

Zebra indexing script misc/migration_tools/rebuild_zebra.pl as a table arg to allow filtering.
When using table=items we should use DISTINCT(biblionumber) to avoid indexing several times the same biblio record when it has several items.
This patch adds DISTINCT(biblionumber) in all cases it does not harm if its already unique.

Test plan :
1) Be sur you have a biblio record with biblionumber 1 with 3 items
2) Run misc/migration_tools/rebuild_zebra.pl -v -b --table items --where="biblionumber=1"
3) Without patch you see "Records exported: 3", with patch only one
4) Check indexing works well

Signed-off-by: Pierre-Marc Thibault <pierre-marc.thibault@inLibro.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>