Keep "Change-Id" unchanged when update patch for golang

• 日本語: golangにコミットする際、"Change-Id" を変えてはいけない

otherwise new “change” (the Gerrit’s counterpart of “pull request” in GitHub) will be created on git codereview mail.

https://go-review.googlesource.com/q/owner:wataash%2540wataash.com

If Change-Id is modified by accident, modify it back manually by git commit --amend. Then git codereview mail will update the original “change”.