version: use OSS repo's version when building.

When building with redo, also include the git commit hash
from the proprietary repo, so that we have a precise commit
that identifies all build info (including Go toolchain version).

Add a top-level build script demonstrating to downstream distros
how to burn the right information into builds.

Adjust `tailscale version` to print commit hashes when available.

Fixes #841.

Signed-off-by: David Anderson <danderson@tailscale.com>

cmd/tailscale/cli/version.go

@@ -36,10 +36,11 @@ func runVersion(ctx context.Context, args []string) error {
 		log.Fatalf("too many non-flag arguments: %q", args)
 	}
 	if !versionArgs.daemon {
-		fmt.Println(version.LONG)
+		fmt.Println(version.String())
 		return nil
 	}
-	fmt.Printf("Client: %s\n", version.LONG)
+
+	fmt.Printf("Client: %s\n", version.String())
 
 	c, bc, ctx, cancel := connect(ctx)
 	defer cancel()