Set GIT_REV from build script

src/build.rs

@@ -1,3 +1,19 @@
+use std::process::Command;
+use std::env;
+
 fn main() {
     println!("cargo:rerun-if-changed=migrations");
+    println!("cargo:rerun-if-changed=.git");
+
+    if env::var("GIT_REV").is_err() {
+        let git_rev: String = Command::new("git")
+            .args(["rev-parse", "HEAD"])
+            .output()
+            .expect("Failed to run git rev-parse")
+            .stdout
+            .try_into()
+            .expect("Failed to parse output from git");
+
+        println!("cargo:rustc-env=GIT_REV={git_rev}");
+    }
 }