respect rust-lang/rust's omit-git-hash

The config.toml file in rust-lang/rust has the omit-git-hash option,
which prevents git information from being embedded into binaries. This
works for most tools, as they rely on the git information provided by
bootstrap through environment variables.

Cargo does its own git detection in its build script though, which
didn't adhere to to that option. This changes that by skipping git
detection whenever bootstrap signals the option is enabled.

build.rs

@@ -50,6 +50,14 @@ fn commit_info() {
     if !Path::new(".git").exists() {
         return;
     }
+
+    // Var set by bootstrap whenever omit-git-hash is enabled in rust-lang/rust's config.toml.
+    println!("cargo:rerun-if-env-changed=CFG_OMIT_GIT_HASH");
+    #[allow(clippy::disallowed_methods)]
+    if std::env::var_os("CFG_OMIT_GIT_HASH").is_some() {
+        return;
+    }
+
     let output = match Command::new("git")
         .arg("log")
         .arg("-1")