This rather large change pulls the "Starting Revision" part of the
new branch dialog into a mega widget that we can use anytime we
need to select a commit SHA-1. To make use of the mega widget I
have also refactored the branch dialog to use the class system,
much like the delete remote branch dialog already does.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
