Deduplicate archive.ar_rev_id

Deduplicate archive.ar_rev_id

Old bugs and such may have left the archive table with multiple rows
using the same ar_rev_id, or rows that also exist in the revision table.
These need to be cleaned up for MCR.

The maintenance script added here will delete rows that appear to be
duplicates of the same change, and will assign new IDs to rows that do
not appear to be duplicates.

Bug: T193180
Change-Id: I39b0825c9469e074ded3df33a4f06a1ef0edb494