EditCounter: prevent edit size counts from counting each other.

[agoujot]

This would also have been solved by a merge of #532 or #551 ; as these implement a cleaner separation between the raw data and the counts we're making. Until we get around to doing that, this is a simpler and smaller fix.

Bug: T400293

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.96%. Comparing base (4b8eb28) to head (ba4849b).
Report is 127 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #560      +/-   ##
============================================
- Coverage     71.30%   70.96%   -0.35%     
- Complexity     1275     1297      +22     
============================================
  Files            46       46              
  Lines          3792     3888      +96     
============================================
+ Hits           2704     2759      +55     
- Misses         1088     1129      +41     

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[agoujot]

(The root problem is that I had assumed that the PHP array counting/filtering functions only read numeric keys. I think I thought that because it does that in JS.)