git config --global core.editor "mate -w"

http://developmentality.wordpress.com/2010/07/21/how-to-make-git-use-textmate-as-the-default-commit-editor-in-git/

Now when you do a git commit without specifying a commit message, TextMate will pop-up and allow you to enter a commit message in it. When you save the file and close the window, the commit will go through as normal. (If you have another text editor you prefer instead, just change the “mate -w” line to the preferred one)

For those curious what the -w argument is about, it tells the shell to wait for the mate process to terminate (the file to be saved and closed).

How to make git use TextMate as the default commit editor « Developmentality

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