[misc] Avoid unnecessary linking when git commit hash not changed (#871)

* [misc] Avoid unnecessary linking when git commit hash not changed

* fix ci

* apply review

misc/generate_commit_hash.py

@@ -6,5 +6,16 @@
 print(f"Building commit {commit_hash}")
 
 output_fn = os.path.join(repo_dir, 'taichi/common/commit_hash.h')
+content = f"#define TI_COMMIT_HASH \"{commit_hash}\"\n"
+
+# First read the file to see if an update is needed
+# This reduces unnecessary file changes/linkings
+if os.path.exists(output_fn):
+    with open(output_fn, 'r') as f:
+        old_content = f.read()
+        if old_content == content:
+            # No update needed
+            exit(0)
+
 with open(output_fn, 'w') as f:
-    f.write(f"#define TI_COMMIT_HASH \"{commit_hash}\"\n")
+    f.write(content)