Include links to the tracking bug and to request creating git repositories. Also
include note how to keep subversion support, if users require this.

It won't warn when git-svn clones are used.

There is no functional change except printing a warning to stderr.

Remove stale code. The CHROMIUM_SRC_URL variable is also defined in gclient.py
so it looks like a copy-paste error.

R=iannucci@chromium.org
BUG=475320

Review URL: https://codereview.chromium.org/1154173003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@295478 0039d316-1c4b-4281-b951-d872f2087c98