x/review/git-codereview: give an error if the Change-Id: line is deleted

[minux]

I think this is the reason why some CLs have more than one Gerrit
CL.

Given that using the same change to open a fresh new gerrit CL is
rarely needed, I think git-codereview should error out if the user
deletes the Change-Id: line (with git change).

And we can add a -f option to override the error (I'm also fine with
let the user git commit --amend to manually remove the Change-Id.)

Of course, git-codereview probably can't detect the case where
the user has used git commit --amend to remove the Change-Id,
but our contribution guideline uses git change, so the proposed
check should catch most problems.

[josharian]

Good idea. How would we propagate the necessary state from git change to the hook? We don't have much context in the hook at this point. In particular, I don't think we have enough context to distinguish this from a commit message coming from a new commit in the same branch (i.e. running regular git commit).

[ysmolski]

IMHO, this cannot be done. I checked what @josharian said. The problem is reduced to: how do we detect that Change_id was removed at all? There is not enough info.