This change makes the "New Review Request" page dynamically
show/hide the relevant form fields for the selected
repository.  This should help clear up some of the confusion
regarding "base path" or "change number" with different
SCMTool backends.

Set up perforce and SVN repositories in my local database.
Switched between different repos on the new page, and saw
the visible form fields change as expected.  Created a
review with said form using the diff posted here.