'git rev-parse <revision> will return same revision result with
newline appended if revision exists. The newline will cause the
following checkout command will fail, so remove the assignment.

BUG=
R=agable@chromium.org, rogerta@chromium.org

Review URL: https://codereview.chromium.org/317783008

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@277615 0039d316-1c4b-4281-b951-d872f2087c98