utils/git: only mutate describe-cache when .git is user-owned

[p-linnane]

set-homebrew-version-from-git deletes and recreates .git/describe-cache whenever it has to fall back to git describe. It did so unconditionally, so brew would rm -rf and mkdir -p inside a Git directory the invoking user does not own, e.g. an installation owned by another user or a .git that has been replaced by a symlink pointing elsewhere.

Only mutate the cache when .git is owned by the current user. Reads are unaffected, so a repository owned by someone else still uses an already populated cache and otherwise just falls back to git describe each run, which is all the cache was ever saving.

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

Claude Code (Opus 5) drafted the implementation and tests; I reviewed the diff, verified the new ownership test fails without the fix and passes with it, and ran brew lgtm + targeted specs.

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[Copilot]

Pull request overview

This PR hardens set-homebrew-version-from-git’s .git/describe-cache handling so brew only deletes/recreates the cache when the repository’s .git directory is owned by the invoking user, avoiding destructive writes inside Git directories owned by other users or reached via symlinked .git.

Changes:

• Add an ownership gate (-O) around all .git/describe-cache mutations (cleanup + rewrite) in set-homebrew-version-from-git.
• Refactor the function to use a shared git_directory variable for .git path accesses.
• Add bash-level RSpec coverage to assert cache writes occur only when .git is user-owned.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
Library/Homebrew/utils/git.sh	Gate .git/describe-cache deletion/creation behind a “.git is user-owned” check to prevent mutating non-owned Git dirs.
Library/Homebrew/test/bash_spec.rb	Add tests validating cache writes when .git is owned and no cache mutation when .git is not owned (via symlink to /).

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[MikeMcQuaid]

Thanks!