not idempotent, slow convergence, and bad converged result

Tested release 1.15.0 with the following

```
public class Foo {

private static final String foo =
"\"aaaaaaaaaaaaaaaaaaaaa\":\"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\","
+ "\"aaaaaaaaaaaaaaaaaaaaa\":\"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\","
+ "\"aaaaaaaaaaaaaaaaaaaaa\":\"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\","
+ "\"aaaaaaaaaaaaaaaaaaaaa\":\"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\","
+ "\"aaaaaaaaaaaaaaaaaaaaa\":\"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\",\"aaaaaaaaaaaaaaaaaaaaa\":\"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\"\n";
}
```

google-java-format would join the last two lines of string and make the last line even longer. Continue invoking google-java-format would keep joining the last part until the whole string is on a single line.

Workaround:
1. remove the last `\n` stops this behavior
2. make the last line shorter than 100 chars stops this behavior

從提供的 Foo 重現開始，在範例上反覆執行 google-java-format。調查結尾換行字元為何會導致持續合併，並驗證格式化會變得冪等、及時收斂，且不使用列出的因應措施也能產生預期結果。

由索引模型根據 Issue 內容生成。