Implement "merge to staging" for a change with all of its dependencies

In gerrit, there is a concept of a change having dependencies on other changes.

It would be nice if there were an option to select "merge to staging with dependencies" on the latest change in a list (the one which depends on all others). This should (atomically) attempt to merge all of them to staging.

It can be argued that in many cases this should not be used, as we try to tell people that all their commits should be self-contained. However I think it is still valid in some cases. For example:

http://codereview.qt.nokia.com/728 - improves the failure message from a failing autotest
http://codereview.qt.nokia.com/729 - fixes the failing autotest, depends on 728

These are conceptually two different things, so I think it's right for them to be in two different commits. But it's fair to want to stage them together: if 729 actually fails to fix the autotest, I want 728 so I get all the info about the failure. Without this feature, the only way to stage them together is to click fast and hope for the best