## Description
Adds a new func to the data.Collection iface:
DoNotMergeItems() tells kopia to skip the
retention of items from prior snapshots when
generating the new snapshot for this collection.
A primary use case for this flag is when a delta
token expires, preventing an incremental lookup
and forcing gc to re-discover all items in the
container.
## Does this PR need a docs update or release note?
- [x] ⛔ No
## Type of change
- [x] 🌻 Feature
## Issue(s)
* #1914
## Test Plan
- [x] ⚡ Unit test
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