Fetch official tap HEAD for worktrees

[MikeMcQuaid]

• Avoid creating official tap worktrees from a stale local branch.
• Preserve source state and offline fallback to the existing HEAD.

---

• Have you followed our Contributing guidelines?
• Have you checked for other open Pull Requests for the same change?
• Have you explained what your changes do? Performance claims (e.g. "this is faster") must include Hyperfine benchmarks.
• Have you explained why you'd like these changes included, not just what they do?
• For bug fixes, have you given step-by-step brew commands to reproduce the bug?
• Have you written new tests (excluding integration tests)? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) locally?

---

• 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.

OpenAI Codex 5.6 Sol xhigh with local review and testing.

---

[Copilot]

Pull request overview

This pull request updates Tap#install to ensure official taps created as worktrees are based on an up-to-date remote commit rather than a potentially stale local branch, while preserving the source checkout’s working state and allowing fallback to the existing HEAD when fetch isn’t possible.

Changes:

• Fetch origin/HEAD in the source tap checkout and use FETCH_HEAD’s commit when creating the detached worktree.
• Add an RSpec regression test ensuring the worktree is created from the fetched remote HEAD without modifying the source checkout state.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
Library/Homebrew/tap.rb	Fetches remote HEAD for source-tap worktree installs and uses the fetched commit for the worktree target.
Library/Homebrew/test/tap_spec.rb	Adds a test covering worktree creation from fetched remote HEAD while preserving source checkout state.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.