Okay, I updated the diff so that it works with trunk svn.

I took the (ugly, static-typed) Java and kinda converted it into prettier pseudo-Python (my Python is sadly kinda rusty, so it might not be perfect):

# The second argument is a list of all the repositories on the ReviewBoard 
# server.
def findRepository( project, repositories ):
    vcs = project.vcs
    localInfo = vcs.getInfo( project.baseDir )
    localUuid = localInfo.repositoryUUID
    # This gives you the directory of the repository you've checked out.  So if
    # the repository is "http://svn/repos" and you checked out
    # "http://svn/repos/trunk/reviewboard", this will return "trunk/reviewboard"
    localRelative = getRelativePath( localInfo.URL, 
                                     localInfo.repositoryRootURL )

    client = ReviewBoardPlugin.getClient( project )

    svnRepositories = filterSvnRepositories( repositories )

    for repository in svnRepositories:
        try:
            info = client.getRepositoryInfo( repository.id )
            remoteUuid = info.repositoryUUID
            if localUuid != remoteUuid:
                continue;

            remoteRelative = getRelativePath( info.URL, info.repositoryRootURL )

            # Now, if the remote relative path is "trunk" and we have
            # "trunk/reviewboard", this will return "reviewboard".  This is the
            # path we need to hand off to reviewboard as the base diff path.
            relative = getRelativePath( localRelative, remoteRelative )

            # This is the same repository, but they're different paths.  The
            # local checkout might be "trunk/reviewboard" and the one on the
            # server might be "branches/2.0".
            if not relative:
                continue;

            return ( repository, relative );
        except Exception:
            pass

    return None;

Yeah, good point.  I'll fix that in a new patch once I understand what you mean on the below comment.

I'm sorry; I don't understand what you mean here.  Do you think there should be more info in the JSON error besides just the error type?

I meant in cases of other exceptions.

I want to see this get in, so to check with what David meant, did you want something like:

try:
    return WebAPIResponse(...)
except NotImplementedError:
    return WebAPIResponseError(request, INVALID_REPO_TYPE)
except Exception, e:
    return WebAPIResponseError(request, something else?)

Yes.

Yes, this is what I meant.  In future, you can just do "except:".  I'll give this one more look-through, and then probably commit it.  Thanks!