Improve truncation logic for Protocol mismatches

https://mypy-play.net/?mypy=latest&python=3.11&gist=430fa5525005889ffb44e2e974a5b67c

from typing import Protocol

class P(Protocol):
    def a(self) -> P: ...
    def b(self) -> P: ...
    def c(self, x: int) -> None: ...
    
class C:
    def a(self) -> C:
        return self
    def b(self) -> C:
        return self
    def c(self, x: str) -> None:
        return None
    
def takes_p(p: P) -> None: ...

takes_p(C())

This gives:

main.py:18: error: Argument 1 to "takes_p" has incompatible type "C"; expected "P"  [arg-type]
main.py:18: note: Following member(s) of "C" have conflicts:
main.py:18: note:     Expected:
main.py:18: note:         def a(self) -> P
main.py:18: note:     Got:
main.py:18: note:         def a(self) -> C
main.py:18: note:     Expected:
main.py:18: note:         def b(self) -> P
main.py:18: note:     Got:
main.py:18: note:         def b(self) -> C
main.py:18: note:     <1 more conflict(s) not shown>
Found 1 error in 1 file (checked 1 source file)

where the actual cause of the conflict is not shown

Reproduce the Protocol mismatch with the linked mypy-play example and inspect how the diagnostic truncates conflicting members. Trace the diagnostic output from this case; done means the message exposes the conflict that currently remains hidden, while retaining appropriate truncation for larger mismatch lists.