[Cosmetic] Commiting a gerrit ChangeId before the commit hook was installed generates confusing message

[strainu]

I was trying to send a new version of a patch to a gerrit server from
a new computer, so I made a change with a ChangeId in the description
and tried to review it:

strainu@emily:~/core> git branch archivebot
strainu@emily:~/core> git checkout archivebot
M pywikibot/page.py
Switched to branch 'archivebot'
strainu@emily:~/core> git diff
strainu@emily:~/core> git add .
strainu@emily:~/core> git commit
[archivebot 282ad24] Update getFileVersionHistoryTable.
 1 file changed, 3 insertions(+), 4 deletions(-)
strainu@emily:~/core> git review -f
Creating a git remote called "gerrit" that maps to:
        ssh://<email address hidden>:29418/pywikibot/core.git
Your change was committed before the commit hook was installed.
Amending the commit to add a gerrit change id.

At this point I ended the transaction, as I was confused by the last
message: I was afraid the ChangeId would have changed, causing the
patch to be attached to another review.

According to a message from the git mailing list, this is a message from git_review/cmd.py

I think git-review should not show this message if the change description
already has a change id, or at least add another message that
clarifies the fact that the change id has not changed.

[strainu (strainu)]

Having looked at the code, I believe the best way forward would be to change that message to: "Amending the commit to add a gerrit change id if none is available."

[strainu (strainu)]

And the patch for that.