Passes through git-fetch's output instead of buffering it.

R=djacques@chromium.org
TBR=agable@chromium.org
BUG=366375

Review URL: https://codereview.chromium.org/264423002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@269080 0039d316-1c4b-4281-b951-d872f2087c98