Rebuild on commit change.

.gitignore

@@ -2,3 +2,4 @@
 /data
 Cargo.lock
 *.pem
+.commit

server/build.rs

@@ -1,5 +1,7 @@
 use std::process::Command;
 
+const F_COMMIT :&str = ".commit";
+
 fn main()
 {
     let output = Command::new("git")
@@ -8,5 +10,12 @@ fn main()
         .expect("Failed to acquire git HEAD");
     let git_hash = String::from_utf8(output.stdout).expect("Invalid UTF-8 output from git HEAD");
     let git_hash = git_hash.trim();
+
+    let previous_hash = std::fs::read_to_string(F_COMMIT).unwrap_or_default();
+    if git_hash != previous_hash {
+        std::fs::write(F_COMMIT, &git_hash).ok();
+    }
+
+    println!("cargo:rerun-if-changed={}", F_COMMIT);
     println!("cargo:rustc-env=GIT_HASH={}", git_hash);
 }