Suppress modified changes for files which weren't actually modified in JSON output.

[ThomasWaldmann]

Port of #8334.

[ThomasWaldmann]

@cr1901 Due to changes in master, these changesets didn't apply well and I had to make some changes, please review.

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.69%. Comparing base (090ee52) to head (1e4314f).
Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
src/borg/archiver/diff_cmd.py	93.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8460      +/-   ##
==========================================
+ Coverage   81.68%   81.69%   +0.01%     
==========================================
  Files          72       72              
  Lines       12950    12961      +11     
  Branches     2345     2347       +2     
==========================================
+ Hits        10578    10589      +11     
  Misses       1708     1708              
  Partials      664      664              

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[cr1901]

these changesets didn't apply well and I had to make some changes

That's fine, because it looks like all but one of the test code changes applied correctly. I marked the single difference in a comment asking for clarification.

All the non-test code looks fine too. Since CI is green for (almost) identical tests, this LGTM pending clarification.