misc/git_hook: propagate shared HOOK_VERSION (#19476)

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>

misc/git_hook/git-hook.go

@@ -14,7 +14,6 @@
 package main
 
 import (
-	_ "embed"
 	"fmt"
 	"log"
 	"os"
@@ -23,9 +22,6 @@ import (
 	"tailscale.com/misc/git_hook/githook"
 )
 
-//go:embed HOOK_VERSION
-var compiledHookVersion string
-
 var pushRemotes = []string{
 	"git@github.com:tailscale/tailscale",
 	"git@github.com:tailscale/tailscale.git",
@@ -47,7 +43,7 @@ func main() {
 	var err error
 	switch cmd {
 	case "version":
-		fmt.Print(strings.TrimSpace(compiledHookVersion))
+		fmt.Print(strings.TrimSpace(githook.HookVersion) + ":0")
 	case "install":
 		err = githook.WriteHooks(hooks)
 	case "pre-commit":