fix(core): fix 2 marker cases in ldml_event_state::emit_difference 🙀

- handle the case where old and new context strings are the same (i.e. no work to do).
- fix pointer arithmetic error in #10356 - the special case where one marker is being replaced by another. The named regression test didn't actually hit this case.

Fixes: bug(core): 'string too long' #11057