This causes git commands to be completely silent (they don't even print
______ running 'git foo bar') if the git command itself is completely silent.
This mirrors the behavior of SVN commands. This is useful for commands like
'gclient diff', which should print no output at all if there is no diff.

R=szager@chromium.org, vadimsh@chromium.org

Review URL: https://codereview.chromium.org/218993009

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@260758 0039d316-1c4b-4281-b951-d872f2087c98