Do get_indexdef calls while the table is already locked.

These calls can require an access share lock on the table, which might
conflict with an existing or later acquires lock. So perform these calls
while we already have an exclusive lock on the table. This unfortuantely
means that we ave to remove the constness of the table parameter to
repack_one_table, as it is not modifying the table object to set up the
indexes.