commit-msg: Insert Change-Id at start of trailers

In older versions of Gerrit the Change-Id field was inserted at the
start of the trailers. Commit 68296f71804feab2e0ae18ae33f834a8a41621e4
simplified the trailers code by using git trailers instead of custom
code but now inserts Change-Id at the end of the trailers section.

A consequence of this is that folks who sign-off their commits using
`git commit -s` now has the sign-off appear first followed by
Change-Id. If the user then runs `git commit -s --amend` to update
the change because the Sign-off-by line is not last, git inserts
a 2nd duplicate Signed-off-by line.

This patch simply restores the previous behaviour of the Gerrit
commit-msg hook where Change-Id would be inserted before the
Sign-off-by line to avoid this issue.

Backported from [1] by Thanh Ha.

[1] https://gerrit-review.googlesource.com/c/gerrit/+/262072

Bug: https://crbug.com/12546
Change-Id: I1406c763a3935761247f6771f55e02367f698e6e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/262352
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: David Pursehouse <dpursehouse@collab.net>