The first take at https://chromiumcodereview.appspot.com/14130006 tried to pass
"--diff-cmd diff", which doesn't work for e.g. Windows cmd users who don't have
a "diff" executable installed.

This take instead passes the "--internal-diff" switch which tells svn to ignore
any diff-cmd set locally and use its internal diff engine at all times.

While implementing this I found that the existing code tried to work around this
problem in a different way, by setting up a bogus config dir.  Since that
doesn't seem to work for me, and shouldn't be necessary with this patch anyway,
removed that code.

Review URL: https://chromiumcodereview.appspot.com/14050007

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@194990 0039d316-1c4b-4281-b951-d872f2087c98