[MERGE] faster extraction of plain texts from knits

[Martin Pool]

Martin Pool has voted tweak.
Status is now: Conditionally approved
Comment:

+    def get_file_text(self, file_id):
+        """Return a string with the text for the file file_id in the 
tree."""
+        raise NotImplementedError(self.get_file_lines)
+

Wrong method name in this exception.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C46AA183B.20403%40arbash-meinel.com%3E