ci: exclude deleted lines from base codecov value

[bougue-pe]

https://docs.codecov.com/docs/commit-status#removed_code_behavior

All components are concerned, open to debate:

• improves (prevents) the case of "false alarm" when removing covered code.
• degrades (hides) the case of legitimate alarm when covered code is replaced by uncovered code (new code must be covered as much as the untouched code).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3a2b863) 27.66% compared to head (009f493) 27.66%.

Additional details and impacted files

@@             Coverage Diff              @@
##                dev    #6307      +/-   ##
============================================
- Coverage     27.66%   27.66%   -0.01%     
  Complexity     2136     2136              
============================================
  Files           990      990              
  Lines        125877   125877              
  Branches       2575     2575              
============================================
- Hits          34825    34824       -1     
- Misses        89562    89563       +1     
  Partials       1490     1490              

Flag	Coverage Δ
core	78.88% <ø> (ø)
editoast	75.57% <ø> (-0.01%)	⬇️
front	8.70% <ø> (ø)
gateway	2.50% <ø> (ø)
railjson_generator	87.43% <ø> (ø)
tests	81.77% <ø> (ø)

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[flomonster]

Looks like a good idea to me.

[shenriotpro]

My gut feeling is that we should be lenient on the "project" checks, but strict on the "patch" checks (i.e. I mostly want to check the added code is tested). Glad to hear other thoughts.