Refactor extract_{summary,description} to use parsed revisions.
The methods to extract the summary and description (used for guessing the

fields for posting, and for matching open review requests with -u) were using

the old revision range, and were a bit ad-hoc. This cleans them up to always

use the parsed revision spec (except in the case of hgsubversion, which is a

pretty "special" case).

Ran unit tests
Posted this change with -g using this code.