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The commit message is from Git, but it is actually the editor that keeps you from quitting. This is because Git uses your default editor, which for a variety of reasons is usually set to vi (it might be something else on your OS, like pico).
To write a commit message and get out of VI, follow these steps:
press i (i for insert)
write your merge message
press esc (escape)
write :wq (write & quit)
then press enter