Fix gitconfig fixture to always write config (Take 4 for #509)

[tony]

Summary

• Fix gitconfig fixture to always write config - removes early return that skipped writing when file exists
• Follow-up to Fix git submodule file:// protocol in test fixture #510, Fix git_repo fixture for submodule file:// protocol #511 for test failure against python 3.14 #509

Problem

The gitconfig fixture had an early return at lines 157-158:

if gitconfig.exists():
    return gitconfig

If ANYTHING creates .gitconfig first (empty or incomplete) - git itself during git init, the nspawn container setup, another fixture - the fixture returns early WITHOUT writing protocol.file.allow=always.

Confirmed by reporter running debug script: empty .gitconfig was created by container/git before the fixture could write the proper config.

Solution

Remove the early return. The fixture now always writes the complete config with protocol.file.allow=always.

Test plan

• test_git_repo_fixture_submodule_file_protocol passes
• All 18 submodule tests pass
• Full test suite passes (565 passed, 1 skipped)
• Arch Linux build test (reporter to verify)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.48%. Comparing base (e3ed829) to head (522e3c4).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #512      +/-   ##
==========================================
+ Coverage   53.46%   53.48%   +0.01%     
==========================================
  Files          38       38              
  Lines        5705     5703       -2     
  Branches     1063     1062       -1     
==========================================
  Hits         3050     3050              
+ Misses       2144     2143       -1     
+ Partials      511      510       -1     

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