Don't re-compile on every cargo run (#4019)

- Add new crate `substrate-build-script-utils` to unify the code of
`node`, `node-template` and `polkadot-node`.
- The `node-cli` build script needs to search upwards for the
`.git/HEAD` file to find it.

substrate/node-template/Cargo.toml

@@ -38,3 +38,4 @@ sr-primitives = { path = "../core/sr-primitives" }
 
 [build-dependencies]
 vergen = "3.0.4"
+build-script-utils = { package = "substrate-build-script-utils", path = "../core/utils/build-script-utils" }

substrate/node-template/build.rs

@@ -7,18 +7,5 @@ const ERROR_MSG: &str = "Failed to generate metadata files";
 fn main() {
 	generate_cargo_keys(ConstantsFlags::SHA_SHORT).expect(ERROR_MSG);
 
-	let mut manifest_dir = PathBuf::from(
-		env::var("CARGO_MANIFEST_DIR").expect("`CARGO_MANIFEST_DIR` is always set by cargo.")
-	);
-
-	while manifest_dir.parent().is_some() {
-		if manifest_dir.join(".git/HEAD").exists() {
-			println!("cargo:rerun-if-changed={}", manifest_dir.join(".git/HEAD").display());
-			return
-		}
-
-		manifest_dir.pop();
-	}
-
-	println!("cargo:warning=Could not find `.git/HEAD` from manifest dir!");
+	build_script_utils::rerun_if_git_head_changed();
 }