libsnapshot: Fix inconsistency in how merge ops are counted.

A recent change to libsnapshot caused us to filter out duplicate COW
ops. The merge consistency check relied on the old method of manually
counting ops, causing it to come up with a different number. Fix this by
using the already computed "official" count.

Bug: 193532829
Test: new test case in vts_libsnapshot_test
      manual test with incremental OTA
Change-Id: I68d1e41f5c140af20a04ba80e3db0780a916ecf8