Correct Windows hook Bash precedence

[Byron]

Tasks

This section is for Byron only. Models continuing this PR must not add, remove, check, uncheck, rename, or reorder checkboxes here.

• refackiew

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Everything below this line was generated by Codex GPT-5.

Created by Codex on behalf of Byron. Byron will review before this is ready to merge.

Summary

Follow-up to #2199 and its post-merge review of the Windows hook Bash resolver. Refs #2198.

The merged resolver still selected the WSL launcher when System32 preceded Git Bash on PATH, and it could not locate Git Bash in the common configuration where PATH contains only Git\cmd. It also skipped an explicitly configured current-directory PATH entry and mocked the resolver in its regression test.

This change first locates the Bash associated with GitPython's selected Git executable. It recognizes standard Git\cmd\git.exe, Git\bin\git.exe, and Git\<platform>\bin\git.exe layouts, then follows gix-path's bin/bash.exe before usr/bin/bash.exe precedence. The PATH fallback excludes candidates below SystemRoot, ignores empty entries according to Windows semantics, and still honors explicitly named directories. The existing bare fallback is retained for nonstandard installations and remains protected from implicit current-directory lookup by safer_popen.

Windows and WSL validation

This machine has Ubuntu on WSL 2, C:\Windows\System32\bash.exe, and Git for Windows.

• Bare bash.exe reported Linux ... microsoft-standard-WSL2.
• git var GIT_SHELL_PATH returned C:/Program Files/Git/usr/bin/sh.exe, which reported MINGW64.
• Under System32;Git\usr\bin, the Resolve Windows hook Bash through PATH #2199 resolver selected System32 and an actual GitPython hook wrote a Linux/WSL marker.
• Under the normal System32;Git\cmd PATH, Bash was absent from PATH, but the corrected resolver selected C:\Program Files\Git\bin\bash.exe; the same end-to-end hook wrote a MINGW64 marker.

Tests

• Regression models System32 before Git\cmd, with Bash absent from PATH, and exercises the real resolver.
• Explicit current-directory PATH entry is accepted while empty entries are ignored.
• Explicit Git\bin\git.exe configuration resolves its sibling Bash, including a relative configured path resolved from the parent process working directory.
• The relative-path test enters its temporary root before constructing the path, because Windows cannot express a relative path across the separate checkout and temporary drives used by CI.
• Focused hook suite: 6 passed.
• Full test/test_index.py: 32 passed, 1 expected xfail, 1 existing xpass.
• Repository-wide Ruff lint and format checks passed using the Python 3.7 target.
• git diff --check passed.
• A standalone Python 3.7 interpreter was unavailable for an additional py_compile run.
• codex review --commit b5ae6d8ee6efee580aa21680786775977131bfde could not inspect the local commit because its Windows sandbox helper was unavailable and fell back to the stale pre-fix PR patch. Its three findings described behavior already corrected in this commit and were verified as false positives against the current source and tests.