Only render the first line of the message in end of line diagnostics

[irh]

set_string_truncated renders the entire string while ignoring newlines, so if the diagnostic's message contains multiple lines then draw_eol_diagnostic produces output like 'first linesecond line'.

An alternative would be to reformat the message to replace newlines with spaces (or something), but I think the first line should be sufficient for end of line diagnostics.

Before:

After: