Improve commit message when updating branch in order to give hint of the underlying commit

[dvojtise]

Currently when updating the submodules in the integration repository the commit message look similar to

Updating submodule gemoc-studio-modeldebugging to track head of branch master

Even if this information is useful, it would be nicer to have commit message that also reuse the last commit message from the branch it follows. This would make the CI build more friendly as it shows the last commit message.