Replacing String with StringBuffer instances in patch_apply

```
We used this DiffMatchPatch code extensively for tracking differences between
versions of database objects. Some of our calls can generate more than 25k
patch strings that need to be applied to large chunks of text. In this instance
we where seeing severe performance issues and inefficient memory management
(even on Java 6).
This patch replaces Strings with StringBuffer and implements more efficient
INSERT & DELETE operations which drastically reduces memory usage for large
text strings.
```

Original issue reported on code.google.com by `chris.p....@gmail.com` on 4 Oct 2011 at 4:37

Attachments:
- [diff_match_patch.java.patch](https://storage.googleapis.com/google-code-attachments/google-diff-match-patch/issue-55/comment-0/diff_match_patch.java.patch)