overcommit --run reports success when Git cannot read the index #909

Reproduced separately on main 50f3e866bef75e3c3be7f92b47b1c45b6e7436ba (0.74.0), macOS 15.6.1, Ruby 4.0.7, Apple Git 2.50.1.

GitRepo.all_files does not check the exit status of either git ls-files command. If Git cannot read the index, both commands fail, but their empty stdout is treated as a valid empty file list. In a scratch repository with a configured TrailingWhitespace hook, the CLI prints Git's fatal errors followed by All pre-commit hooks passed, and exits 0.

Reproduction

1. In a new disposable repository, configure only TrailingWhitespace with include: ['**/*.rb'] and on_warn: fail, and approve the fixture configuration with overcommit --sign.
2. Create and stage normal.rb containing a trailing-space violation. Confirm overcommit --run exits 65.
3. Replace only that disposable repository's .git/index with invalid synthetic bytes. No user repository or live index is needed.
4. Run overcommit --run again.

Observed stderr:

fatal: .git/index: index file smaller than expected
fatal: .git/index: index file smaller than expected

Observed stdout ends with:

All pre-commit hooks passed

Exit status is 0. A valid empty index also exits 0, so callers cannot distinguish the two states using the process status.

Expected behavior and scope question

When Git cannot enumerate the files to check, --run should report an enumeration error and return a nonzero status rather than success. GitRepo.list_files already checks command success and raises an Overcommit error; all_files currently uses unchecked backticks.

I checked #524: it concerned restoring the working tree/stash after normal pre-commit execution and did not provide a reproducible cause of index corruption. This report neither attributes index corruption to Overcommit nor asks it to repair a broken index. It is specifically about the standalone --run exit status once enumeration fails.

Would a separate change checking both enumeration command results be in scope? The controls would cover a valid empty index, normal violation, deleted tracked files, and Git enumeration failure. I would not combine it with filename parsing, stash handling, or index recovery.