Add a regression test for git commit -a index corruption

[j178]

closes #1815
closes #1786

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.72%. Comparing base (7116100) to head (a6dbdb1).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1819   +/-   ##
=======================================
  Coverage   91.72%   91.72%           
=======================================
  Files          98       98           
  Lines       20055    20055           
=======================================
+ Hits        18395    18396    +1     
+ Misses       1660     1659    -1     

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[Copilot]

Pull request overview

This PR updates prek’s integration test suite around git commit -a hook execution to better normalize snapshot output and to document/reproduce a currently-known failure mode involving GIT_INDEX_FILE.

Changes:

• Extend the git_commit_a snapshot filters to normalize an additional hash value.
• Add a new Unix-only regression/repro test that captures the current failure when a hook writes to a temp git index during git commit -a.