We cannot run svn log --xml interactively because the authentication

prompts are intermixed with the svn log output. This results in

exceptions when attempting to parse the XML and leads to cryptic error

messages.
This patch replaces all svn log --xml usage with non-interactive usage

and checks for a specific error code to determine if there was an

authentication error. Authentication errors are written to the console

with directions on how to rectify them using the SVN authenticaiton

command line options.

Ran unit tests.
Ran rbt diff in an SVN repository with specific revisions with a

remote repository over HTTP with authentication. RBTools exited with

an error that prompted me to enter my SVN credentials on the

command-line.