gpush: introduce option to add missing Change-Id footers

it's not exactly rare to try to push something to Gerrit, only to
discover that the commit-msg hook wasn't installed. of course one can
fix that and then simply run 'git rebase -i' and 'reword' each commit,
but that gets a bit tedious with a lot of Changes ...

Change-Id: I5a47f615452cc929dacbe621cc3116d2d8cc7b2d
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>