misc: add install-git-hooks.go and git hook for Change-Id tracking

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- post-checkout: warns when the hook binary is outdated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>

1 files changed, 1 insertions, 1 deletions

diff --git a/docs/commit-messages.md b/docs/commit-messages.md
index 832a4cc9e..57047a803 100644
--- a/docs/commit-messages.md
+++ b/docs/commit-messages.md
@@ -74,7 +74,7 @@ For the body (the rest of the description):
 - blank line after the subject (first) line
 - the text should be wrapped to ~76 characters (to appease git viewing tools, mainly), unless you really need longer lines (e.g. for ASCII art, tables, or long links)
 - there must be a `Fixes` or `Updates` line for all non-cleanup commits linking to a tracking bug. This goes after the body with a blank newline separating the two. A pull request may be referenced rather than a tracking bug (using the same format, e.g. `Updates #12345`), though a bug is generally preferred. [Cleanup commits](#is-it-a-cleanup) can use `Updates #cleanup` instead of an issue.
-- `Change-Id` lines should ideally be included in commits in the `corp` repo and are more optional in `tailscale/tailscale`. You can configure Git to do this for you by running `./tool/go run misc/install-git-hooks.go` from the root of the corp repo. This was originally a Gerrit thing and we don't use Gerrit, but it lets us tooling track commits as they're cherry-picked between branches. Also, tools like [git-cleanup](https://github.com/bradfitz/gitutil) use it to clean up your old local branches once they're merged upstream.
+- `Change-Id` lines should be included in commits. You can configure Git to do this for you by running `./tool/go run misc/install-git-hooks.go` from the root of the repo. This was originally a Gerrit thing and we don't use Gerrit, but it lets tooling track commits as they're cherry-picked between branches. Also, tools like [git-cleanup](https://github.com/bradfitz/gitutil) use it to clean up your old local branches once they're merged upstream.
 - we don't use Markdown in commit messages. (Accidental Markdown like bulleted lists or even headings is fine, but not links)
 - we require `Signed-off-by` lines in public repos (such as `tailscale/tailscale`). Add them using `git commit --signoff` or `git commit -s` for short. You can use them in private repos but do not have to.
 - when moving code between repos, include the repository name, and git hash that it was moved from/to, so it is easier to trace history/blame.