When a Git-versioned model project is being viewed in the Trace Matrix, it can be compared to other versions of the same project in Git in multiple ways offered in the context menu.
Right-click on the "models" project in the Package Exporer. The project can be compared to another using a Git tag or branch (grouped in Remote Tracking in Git's native dialog)
Right-click on the "models" project in the Package Exporer. This command lets you compare the project to a selected Git commit. Note: You may want to choose whether all commits or only those of the checked-out branch are displayed in the dialog. This among many other options for this dialog can be set in Eclipse's preferences, Team -> Git -> History
This command lets you compare multiple projects selected in the Project Explorer pane. It allows for traces contained in an abritray number or kind of projects to be compared, and is not recommended for non-expert users.