Thanks for the post and it seems like when you moved to the new computer the git was not configured? The fact that you switched Macs and upgraded Xcode/macOS versions is a strong indicator, for me, that the issue stems from the environment differences or a configuration mismatch.
Open your Terminal application and navigate to your project directory.
What do you get when you request the git status? git status Does git status accurately reflect the staged changes you see in Xcode? Are there any "untracked files" that Xcode might be confused about? Are there any error messages?
There are a few things you can do in the terminal to make sure git was installed and configured correctly:
git commit -m "Test commit message from terminal"
git diff --cached
In Xcode, go to the Navigator pane (left-hand side). Select the "Source Control" tab (the icon with branches or waves). Does it show any errors or warnings related to your repository?
Check the configuration on git with using the correct username and email address.
To check:
git config --global user.name
git config --global user.email
git config --global user.name "Your Name"
git config --global user.email "you email address"
The error messages you get from the Terminal will be key to pinpointing the exact cause. Good luck!
Albert Pascual
Worldwide Developer Relations.