misc/git_hook: propagate shared HOOK_VERSIONfserb/githook-version

Move HOOK_VERSION into the githook package and export it as
githook.HookVersion, so tailscale/corp can reference it via
the shared-code bump instead of having to bump HOOK_VERSION
by hand.

New launcher.sh composes the wanted version from 2 sources:
the shared HOOK_VERSION and an optional repo local version,
misc/git_hook/HOOK_VERSION, for repo-specific config bumps.

Updates tailscale/corp#40381

Change-Id: I7cf16889ba53cb564cc2df7dfd7588748f542c55
Signed-off-by: Fernando Serboncini <fserb@tailscale.com>

1 files changed, 9 insertions, 8 deletions

diff --git a/misc/git_hook/README.md b/misc/git_hook/README.md
index 81c15dc58..49d768937 100644
--- a/misc/git_hook/README.md
+++ b/misc/git_hook/README.md
@@ -22,13 +22,14 @@ script after a built-in hook. For example, put a custom check in
 only if the built-in hook succeeds; failure aborts the git operation.
 
 
-## Changing the shared code
+## Version bumps
 
-When you change anything under `githook/` or `launcher.sh`, bump
-`HOOK_VERSION` in the same commit so every dev auto rebuilds on their next
-git operation.
+The launcher rebuilds when the installed binary's version differs from
+the concatenation of two files:
 
-Because `tailscale/corp` imports `githook/`, also plan the downstream
-update: after landing here, bump corp's `tailscale.com` dependency and
-bump corp's own `misc/git_hook/HOOK_VERSION` on a separate commit. Both are
-required.
+* `githook/HOOK_VERSION` (shared): bump when changing anything under
+  `githook/` or `git-hook.go`. Downstream repos pick it up after
+  bumping their `tailscale.com` dependency.
+* `misc/git_hook/HOOK_VERSION` (repo-local, optional): bump to force a
+  rebuild for repo-specific config changes without touching the shared
+  version. This repo does not use one.