Do not emit newline when running commands if the cursor is on its own line

If the cursor has been wrapped to the last line, and is the only thing
on that line, do not emit a newline when executing a command.

Fixes #6826

src/screen.cpp

@@ -1222,3 +1222,10 @@ void screen_force_clear_to_end() {
 }
 
 screen_t::screen_t() : outp_(outputter_t::stdoutput()) {}
+
+bool screen_t::cursor_is_wrapped_to_own_line() const {
+    // Note == comparison against the line count is correct: we do not create a line just for the
+    // cursor. If there is a line containing the cursor, then it means that line has contents and we
+    // should return false.
+    return actual.cursor.x == 0 && static_cast<size_t>(actual.cursor.y) == actual.line_count();
+}