Have disable merge bases turn all merge
bases into assist bases instead of
completely removing them. This allows
us to continue getting benefits of
kopia assisted incrementals and fixes
the not having the backup models for
details merging
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#### Does this PR need a docs update or release note?
- [x] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [ ] ⛔ No
#### Type of change
- [ ] 🌻 Feature
- [x] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup
#### Issue(s)
* fixes#4178
* #3943
#### Test Plan
- [ ] 💪 Manual
- [x] ⚡ Unit test
- [x] 💚 E2E