The "test-diff" command:

Compute an "external diff" between two files. By "external diff" we mean a diff between two disk files that are not necessarily under management. In other words, this command provides a mechanism to use Fossil's file difference engine on arbitrary disk files. See the "diff" command for computing differences between files that are under management.

This command prints the differences between the two files FILE1 and FILE2. all of the usual diff formatting options (--tk, --by, -c N, etc.) apply. See the "diff" command for a full list of command-line options.