Detect prerequisite branches/merge proposals automatically

[Jamu Kakar]

When branch A depends on branch B, the correct way to file a merge
proposal for B is to specify A as a prerequisite. The controls to
specify the prerequisite are hidden in an 'Advanced' section of the
merge proposal creation form. Partly because of this, and partly
because it's not a very common operation, specifying the prerequisite
is often forgotten. The result is a merge proposal with a diff that
doesn't actually represent the change that needs to be reviewed.

When this happens the branch author either does nothing and reviewers
need to figure out what happens, or the branch author has to jump
through some hoops to refile their proposal.

It would be really great if Launchpad could just detect that branch A
was a prerequisite of branch B. I suppose, this could only happen if
there was already for a merge proposal for branch A, but at least in
my uses, I usually file merge proposals in order of branch dependency.