The "diff" command:

Usage: fossil diff|gdiff ?OPTIONS? ?FILE1? ?FILE2 ...?

Show the difference between the current version of each of the FILEs
specified (as they exist on disk) and that same file as it was checked
out.  Or if the FILE arguments are omitted, show the unsaved changed
currently in the working check-out.

If the "--from VERSION" or "-r VERSION" option is used it specifies
the source check-in for the diff operation.  If not specified, the 
source check-in is the base check-in for the current check-out.

If the "--to VERSION" option appears, it specifies the check-in from
which the second version of the file or files is taken.  If there is
no "--to" option then the (possibly edited) files in the current check-out
are used.

The "-i" command-line option forces the use of the internal diff logic
rather than any external diff program that might be configured using
the "setting" command.  If no external diff program is configured, then
the "-i" option is a no-op.  The "-i" option converts "gdiff" into "diff".

The "-N" or "--new-file" option causes the complete text of added or
deleted files to be displayed.

Options:
  --brief             Show filenames only
  --context|-c N      Use N lines of context 
  --from|-r VERSION   select VERSION as source for the diff
  -i                  use internal diff logic
  --new-file|-N       output complete text of added or deleted files
  --to VERSION        select VERSION as target for the diff
  --side-by-side|-y   side-by-side diff
  --width|-W N        Width of lines in side-by-side diff