This change replaces the old "Update Diff" dialog with a new version that's

based on the pre-commit workflow from the "New Review Request" page. This makes

it so that the general case is as simple as selecting or drag-and-dropping a

diff file, and other questions (such as the basedir or parent diff) are only

asked if necessary.
The PreCommitView from the New Review Request page has been generalized into

UploadDiffView, which both the PreCommitView and UpdateDiffView extend. For the

most part, the only difference between these are the specific templates that

are used, and the fact that the update dialog uses $.fn.modalDlg around

this.$el.
This eliminates one of our two remaining uses of $.formDlg and gReviewRequest,

getting us that much closer to a clean javascript codebase.

Went through all the old scenarios with the "New Review Request" page to test

  for regressions.
Updated a diff on a review request and saw the resulting change.
Updated a diff on a review request that required a parent diff, specified

  that parent diff, and saw the resulting change.
Tried to update a diff on a review request with a patch file that had short

  revs for git and saw the appropriate error.
Updated a diff on a review request using an svn repository and was prompted

  for the base directory.