Tell download tools/hooks/commit-msg mannually if it looks broken.

chrome-internal-reviews.googlesource.com requires
authentication to access tools/hooks/commit-msg, and
we'll get Google Sign-in page rather than expected shell script.

R=szager@chromium.org,maruel@chromium.org,ilevy@chromium.org
BUG=

Review URL: https://codereview.chromium.org/87173002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@237454 0039d316-1c4b-4281-b951-d872f2087c98