isUpdateOf check field presence before comparison.

isUpdateOf is passed an "other" changeish that may be a Ref which does
not have number and patchset fields which are compared against in
Change's isUpdateOf method. Check that fields exist before comparisons
are made.

Change-Id: Ifb5a36dcb0377f496e6b0707ae27810b96f2c752
Reviewed-on: https://review.openstack.org/22159
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins