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/cli/Cargo.toml

@@ -91,6 +91,7 @@ tempfile = "3.1.0"
 
 [build-dependencies]
 substrate-cli = { package = "substrate-cli", path = "../../core/cli" }
+build-script-utils = { package = "substrate-build-script-utils", path = "../../core/utils/build-script-utils" }
 structopt = "0.3.3"
 vergen = "3.0.4"
 
@@ -118,7 +119,7 @@ cli = [
 ]
 wasmtime = [
 	"cli",
-    "node-executor/wasmtime",
-    "substrate-cli/wasmtime",
-    "substrate-service/wasmtime",
+	"node-executor/wasmtime",
+	"substrate-cli/wasmtime",
+	"substrate-service/wasmtime",
 ]

substrate/node/cli/build.rs

@@ -21,9 +21,9 @@ use vergen::{ConstantsFlags, generate_cargo_keys};
 
 fn main() {
 	build_shell_completion();
-	generate_cargo_keys(ConstantsFlags::all())
-		.expect("Failed to generate metadata files");
-	println!("cargo:rerun-if-changed=.git/HEAD");
+	generate_cargo_keys(ConstantsFlags::all()).expect("Failed to generate metadata files");
+
+	build_script_utils::rerun_if_git_head_changed();
 }
 
 /// Build shell completion scripts for all known shells