How to exit a git merge asking for commit message?

This is depend on the editor you're using.

If vim you can use ESC and :wq or ESC and Shift+zz. Both command save file and exit.

You also can check ~/.gitconfig for editor, in my case (cat ~/.gitconfig):

[user]
    name = somename
    email = [email protected]
[core]
    editor = vim
    excludesfile = /home/mypath/.gitignore_global
[color]
  ui = auto
  # other settings here