How to restrict gitk to show only branch/merge commits?

[Josef Wolf <jw@raven.inka.de>]

Hello,

I would like gitk to show me only the heads, branch-points and
merge-points of branches.

For example, given a history like this:

                   d...e--F--g...h--H
                  /        \
 a1--a2...an--b--C          N--o...p--P
                  \        /
                   i...j--K--l...m--M

I would like to see only

 - a1     because it has no parent
 - C,F,K  because they are reachable from multiple heads
 - N      because it has multiple parents
 - H,M,P  because they are heads

Ideally, there would be a possibility to add some "context" to the
list of commits to be shown: with $context==1, all the commits shown
in the above history would be shown, but the commits indicated by
the triple-dots would be omitted.

The reason is that I have some repositories with several really long
linear history.  It is hard to get a global overview about the branch
history if you have to scroll constantly.

Is something like that possible with gitk?