Add env variable to suppress the notice about deprecated SVN
Add env variable to suppress the notice about deprecated SVN

Also removed part of the message that encourages users to disable updates to
depot_tools. If the users do not enable updates, it prevents landing changes into
project repositories that depend on having latest version of the depot_tools, e.g.
https://codereview.chromium.org/1152823005/.

R=akuegel@chromium.org
TBR=maruel@chromium.org, iannucci@chromium.org
BUG=475320

Review URL: https://codereview.chromium.org/1162153004

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@295486 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools