Merge lists using a for-loop instead of unionBy

[kinow]

These changes close #778

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Appropriate tests are included (unit and/or functional).
• No change log entry required (why? e.g. invisible to users, not released yet).
• No documentation update required.

[codecov-commenter]

Codecov Report

Merging #779 (6899398) into master (d86f687) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #779      +/-   ##
==========================================
+ Coverage   90.97%   90.98%   +0.01%     
==========================================
  Files          84       84              
  Lines        1673     1675       +2     
  Branches      105      105              
==========================================
+ Hits         1522     1524       +2     
  Misses        121      121              
  Partials       30       30              

Flag	Coverage Δ
e2e	77.85% <66.66%> (-0.04%)	⬇️
unittests	81.66% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/graphql/merge.js	100.00% <100.00%> (ø)

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[kinow]

This is essentially what was happening when merging the subscriptions; see linked issue for the complete diff.

[kinow]

To review:

1. Go to master, build it for development
2. Open a tree view
3. Add a second tree view, watch either Network tab for the stop/start messages, or see if the UI clears the views and re-populates them

Then switch to this branch and try that again. The subscription must not change since you are adding a second tree view.

[oliver-sanders]

Code looks good, haven't tested yet (my clones are in a mess).

[oliver-sanders]

Tested, the subscription wasn't stopped / restarted 👍.