Did you solve this problem?
I had a similar problem when I update macOS from 10.15 to 11.5.2. Both use same Xcode is 12.4 and project file.
The source control shows git logs and differences, but main editor (Code review) doesn't show the differences while the 'M' is marked at the file.
The right editor window (Code review) shows the "Unable to load revisions" error.
I copy the project folder from the previous volume which was macOS 10.15 is installed to the new volume. In the new volume macOS 11.5.2 and Xcode 12.4 are installed. Then the problem is gone.
I hope this helps someone see the same problem.
auchino
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