graft: use a real conflict for the tests
One of the graft tests grafts a changeset that changes a file's
content from 'a' to 'b' onto a branch that has changed the file's
content from 'a', via 'b', and then back to 'a' again. To prepare for
not considering this a file in need of merging, let's use 'c' as the
file's new content to make sure it has to be considered
conflicting.
There's a second similar case further down where an ancestor is
grafted. Make sure that is also considered a conflict.