fossil: -c 0 currently (<=fossil-2.23) does not work as expected (uses default number of context lines)

[berndf]

Just for the record: For fossil, 'fossil diff --unified -c 0 -- %f' is used but '-c 0' is currently just ignored by fossil, so the change marks shown by signify span 5 more lines than intended on either side of the actual change.

This is just to note the discussion at https://fossil-scm.org/forum/forumpost/b5e4e80a63 which lets me hope that '-c 0' will be supported in the future.