feat: filter entry list from github before reading files

[KaneFreeman]

Related to decap PR: decaporg/decap-cms#6898

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[codecov]

Codecov Report

Merging #868 (3bf9f23) into main (f5f79b3) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #868      +/-   ##
==========================================
- Coverage   51.43%   51.38%   -0.05%     
==========================================
  Files         225      225              
  Lines        9663     9672       +9     
  Branches     2594     2598       +4     
==========================================
  Hits         4970     4970              
- Misses       4313     4321       +8     
- Partials      380      381       +1     

Files Changed	Coverage Δ
packages/core/src/backend.ts	12.50% <0.00%> (-0.21%)	⬇️
...ckages/core/src/backends/github/implementation.tsx	43.95% <0.00%> (-0.25%)	⬇️