Fix git hooks in worktrees

[alexh-tigera]

Description

Allow git hooks to work when the operator repo is a worktree.

Release Note

TBD

For PR author

• [ ] Tests for change.
• [ ] If changing pkg/apis/, run make gen-files
• [ ] If changing versions, run make gen-versions

For PR reviewers

A note for code reviewers - all pull requests must have the following:

• Milestone set according to targeted release.
• Appropriate labels:
  • kind/bug if this is a bugfix.
  • kind/enhancement if this is a a new feature.
  • enterprise if this PR applies to Calico Enterprise only.

[Copilot]

Pull request overview

Enables git hooks and make pre-commit to function correctly when the operator repo is checked out as a git worktree (where .git is a file pointing elsewhere rather than a directory).

Changes:

• install-git-hooks now uses git rev-parse --git-common-dir to locate the hooks directory and creates it if missing, supporting both regular and worktree checkouts.
• Makefile computes GIT_COMMON_DIR from git rev-parse --git-common-dir, mounts it into the container when it lives outside CURDIR/.git, and uses it to read objects/info/alternates.

Reviewed changes

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

File	Description
install-git-hooks	Install hooks into the common git dir (handles worktrees) and create the hooks dir if absent.
Makefile	Add GIT_COMMON_DIR + WORKTREE_GIT_MOUNT and use them in the pre-commit target so the containerized hook can access git metadata in worktrees.

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