** 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.
**
** If the "--checkin VERSION" option shows the changes made by
** check-in VERSION relative to its primary parent
** The "--checkin VERSION" option shows the changes made by
** check-in VERSION relative to its primary parent.
**
** 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