Allow git rebase --abort to fail.
Allow git rebase --abort to fail.

Specifically, do so because certain git rebase invocations flat out
fail before even starting the rebase operation.  In that case, there
is nothing to abort.

This is a regression introduced in b0bc3458f2b92bf363.

BUG=None
TEST=cbuildbot x86-generic-paladin

Change-Id: I84a7f50dce44bd7955f0b2925b5f8667eddc3742
Reviewed-on: https://gerrit.chromium.org/gerrit/20420
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Reviewed-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>
Tested-by: Brian Harring <fdc2d0a2e2340b623587cee5ea8dff425d1eb1f3@chromium.org>

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