Restoring an older public API method.

- I changed what I thought was a private API, but it turns out that
  people used Hunk#unshift. Because the Hunk#merge functionality is
  safer and more correct, I have provided Hunk#unshift as an alias to
  Hunk#merge.
- Note that Hunk#unshift should be considered deprecated and the alias
  will be removed in a future major version of Diff::LCS.
History update for release; version bump.