add git hash

build.rs

@@ -0,0 +1,7 @@
+use std::process::Command;
+fn main() {
+    // note: add error checking yourself.
+    let output = Command::new("git").args(&["rev-parse", "HEAD"]).output().unwrap();
+    let git_hash = String::from_utf8(output.stdout).unwrap();
+    println!("cargo:rustc-env=GIT_HASH={}", git_hash);
+}