Backport: diff: reduce memory consumption, fix is_hardlink_master

[taandrighetti]

Backports #6295

[ThomasWaldmann]

Thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.88%. Comparing base (ad50b2c) to head (345f014).
⚠️ Report is 164 commits behind head on 1.1-maint.

Additional details and impacted files

@@              Coverage Diff              @@
##           1.1-maint    #6300      +/-   ##
=============================================
- Coverage      79.89%   79.88%   -0.01%     
=============================================
  Files             27       27              
  Lines          10494    10494              
  Branches        2149     2149              
=============================================
- Hits            8384     8383       -1     
+ Misses          1589     1588       -1     
- Partials         521      523       +2     

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[ThomasWaldmann]

Interesting. Codecov comments on merged PRs 4 years later.