Sync in additive mode by default

[goosemania]

re #4449
https://pulp.plan.io/issues/4449

[codecov]

Codecov Report

Merging #3898 into master will decrease coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3898      +/-   ##
==========================================
- Coverage   67.54%   67.43%   -0.11%     
==========================================
  Files          64       64              
  Lines        2822     2822              
==========================================
- Hits         1906     1903       -3     
- Misses        916      919       +3

Impacted Files	Coverage Δ
pulpcore/app/serializers/repository.py	95.27% <ø> (ø)	⬆️
pulpcore/content/handler.py	60.74% <0%> (-2.23%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 45f5ee3...ae7e903. Read the comment docs.