up vote 97 down vote accepted I think it's not Git error message, it's the editor as git uses your default editor. to solve this.
1-press "i" 2-write your merge message 3-press "esc" 4-write ":wq" then press enter
http://stackoverflow.com/questions/19085807/please-enter-a-commit-message-to-explain-why-this-merge-is-necessary-especially