Skip writing reflection_generated.h if not changed (#5776)

reflection/generate_code.sh

@@ -15,4 +15,16 @@
 # limitations under the License.
 set -e
 
-../flatc -c --cpp-std c++0x --no-prefix -o ../include/flatbuffers reflection.fbs
+tempDir="../include/flatbuffers/.tmp"
+originalFile="../include/flatbuffers/reflection_generated.h"
+newFile="$tempDir/reflection_generated.h"
+
+../flatc -c --cpp-std c++0x --no-prefix -o $tempDir reflection.fbs
+
+if [ -f "$newFile" ]; then
+  if ! cmp -s "$originalFile" "$newFile"; then
+    mv $newFile $original
+  fi
+  rm $newFile
+  rmdir $tempDir
+fi