Recently in a discussion on the Git mailing list, Linus admitted (and I agreed) that this was one of the design mistakes we made early in the history of Git. And in 1.7.10 and later, the git merge command that is run in an interactive session (i.e. both its standard input and its standard output connected to a terminal) will open an editor before creating a commit to record the merge result...
a new command line option will be introduced that controls this functionality:
...it will be sufficient to run the command with the --no-edit option and accept the canned commit log message without editing.