Make git-nav-downstream synchronous
Make git-nav-downstream synchronous

Currently git-nav-downstream invokes 'git checkout' in a subprocess, but
exits without waiting for the checkout to complete. Instead wait for the
subprocess to complete.

BUG=376101

Review URL: https://codereview.chromium.org/509163002

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@291701 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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