How to submit a patch to launchpad properly

Hello,

Let's say I'm working on a project whose trunk branch is hosted in launchpad (it has thousands of revisions) and that I maintain a local branch that is basically a mirror of the branch in launchpad.

When I want to fix a bug, I do the following:
1) Create a new branch locally from my local trunk
2) Fix the bug (small number of commits)
3) Push the changes to a new launchpad branch
4) Create a merge proposal against trunk branch in launchpad

The problem is that when pushing the changes to launchpad (3), despite the branch is stacked against trunk (which is correct, since they both have a common history), it looks like the whole history has to be uploaded to launchpad (which may take quite a long time) even when the difference against trunk is just a few commits.

Is there a smarter way to to submit a patch and create a merge proposal in launchpad?

It would really nice, for example, to be able to create a new branch in launchpad based on trunk (not an empty branch) so when I submit my changes, only the new changes have to be uploaded. This would be speed up the process since local branch operations are fast.

Best regards,
    Javier