Cleanup failed rebases in cros_build_lib.py.
Cleanup failed rebases in cros_build_lib.py.

If a rebase fails we should be nice and git rebase --abort so that we won't
leave the tree in a busted state.

BUG=chromium-os:16098
TEST=ran prebuilt test suite

Change-Id: I60c13e9d0e436f8091b5aff4546d2966daed50c1
Reviewed-on: http://gerrit.chromium.org/gerrit/2302
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Reviewed-by: Scott Zawalski <d0bc454f32175e2361d26fbdb5930ff6be11acc5@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

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