I finally solved the problem by removing ~/Library/Caches/com.apple.amp.itmstransporter folder. For safety, it might be better to rename by appending a suffix such as .old. Then, I uploaded successfully by xcrun altool --upload-app, which recreates the removed cache folder automatically. Copied from https://stackoverflow.com/a/64373421/1107242

When ⌘ + 2 to view the Source Control panel, branch names can be displayed completely. It might be caused by the same bug.