svn revert is broken in too many ways to work around it. I just discovered --revision BASE and it does everything I need.

BUG=none
TEST=gclient revert ignores externals, work with switched directories, work with files with inconsistent line endings (sigh)

Review URL: http://codereview.chromium.org/209052

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@26707 0039d316-1c4b-4281-b951-d872f2087c98