Add retries to git remote update in prebuilt.py.
Add retries to git remote update in prebuilt.py.

We often see failures in prebuilt.py due to git remote update failing.
This is a server issue, but adding retries will help reduce the incidence
of this failure.

BUG=chromium-os:19708
TEST=Unit tests. Cbuildbot run.

Change-Id: I10529fa22a078f968b2fc54e8afc84a57d31992b
Reviewed-on: http://gerrit.chromium.org/gerrit/6712
Reviewed-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

coreos/chromite,coreos/chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,chadversary/chromiumos.chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite