Add misc/install-git-hooks.go and misc/git_hook/ to the OSS repo,
adapted from the corp repo. The primary motivation is Change-Id
generation in commit messages, which provides a persistent identifier
for a change across cherry-picks between branches.
The installer uses "git rev-parse --git-common-dir" instead of go-git
to find the hooks directory, avoiding a new direct dependency while
still supporting worktrees.
Hooks included:
- commit-msg: adds Change-Id trailer
- pre-commit: blocks NOCOMMIT / DO NOT SUBMIT markers
- pre-push: blocks local-directory replace directives in go.mod
- post-checkout: warns when the hook binary is outdated
Also update docs/commit-messages.md to reflect that Change-Id is no
longer optional in the OSS repo.
Updates tailscale/corp#39860
Change-Id: I09066b889118840c0ec6995cc03a9cf464740ffa
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
