Enter custom merge commit message when merging pull requests

If you look at merge commits, it is difficult to get an idea of what was merged when all commits look like this:

Merge pull request #N in project/repository from user/repository:branch into master.
Merge pull request #K in project/repository from user/repository:branch into master.
Merge pull request #J in project/repository from user/repository:branch into master.

I have no problem with making this a default merge commit message, but I'd like to override it so I can see messages like:

Fixes issue #N.
Adds feature XYZ.
...