[MERGE] Be more efficient with sha operations during commit

[Ian Clatworthy]

Robert Collins wrote:
> This saves 10 seconds or so on an initial commit of a mozilla tree by
> using sha_string, rather than sha_strings.

bb:tweak

> As part of it I've removed an unused function from the _KnitData class,

As _KnitData is a private class, it's not strictly needed though
mentioning in NEWS that add_record was removed would be nice.

> @@ -908,13 +901,20 @@
>          if delta:
>              options.append('line-delta')
>              store_lines = self.factory.lower_line_delta(delta_hunks)
> +            size, bytes = self._data._record_to_data(version_id, digest,
> +                store_lines)
>          else:
>              options.append('fulltext')
> +            # get mixed annotation + content and feed it into the
> +            # serialiser.
>              store_lines = self.factory.lower_fulltext(content)
> +            size, bytes = self._data._record_to_data(version_id, digest,
> +                store_lines)

Setting of size & bytes is common to both branches of the if statement
so pull it out as common code.

Ian C.