Fix a bug in scm.SVN.DiffItem()

Reduce variable aliasing, fix alignments, etc.

Review URL: http://codereview.chromium.org/2673002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@48943 0039d316-1c4b-4281-b951-d872f2087c98