Use the --no-edit option, you can read about it in the documentation.
Note that using the default message is discuraged, since it provides no meaningful information about the changes introduced with this merge.
On a sidenote: To continue merging you probably have to close the editor.
If you have a git version prior to 1.7.8 there is still a way to achieve what you want by using the env command.
env GIT_EDITOR=: git merge <ref-you-want-to-merge>
For easier usage you could create an alias.
git config --global alias.merge-no-edit '!env GIT_EDITOR=: git merge'
Which then can be used using git merge-no-edit <ref-you-want-to-merge>.