The "xdiff" 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.
This command used to be called "test-diff". The older "test-diff" spelling still works, for compatibility.