The subvertpy backend for post-commit review request creation had a bug where

it was passing in a string for the base revision to the diff method. This is

fixed easily enough by ensuring that revisions are ints.
While I was doing this, I found that trying to create post-commit review

requests for commits which are in a protected part of the repository (such as

security fixes in the ASF repo) would show the user a dialog that says "HTTP

Error 500". I've converted exceptions in the diff method to SCMError, which

will at least log and return something slightly less ugly to the user.

Created some post-commit review requests for commits in the WordPress and ASF

repositories.