The comment made by @TheComet is great but for anyone else who comes to this post and is stil having this issue, you will need to set up your "user.name" and "user.email" via the command line.
This is required because Xcode needs to see who the author is and then it will allow you to set up a commit message.
Example commands to run (assuming "git" is already installed):
git config --global user.email "user@domain.tld"
git config --global user.name "myusername"
No need to restart Xcode, just close the commit window and re-open it, voila!
varcharfoobar
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