dont create new revision if image is unchanged

## In what area(s)?
Classifications:
/kind bug

## What version of Knative Client?
Version: v20190828-local-f53ebd8
Build Date: 2019-08-28 09:06:01
Git Revision: f53ebd8-dirty
Dependencies:
- serving: v0.6.0
- eventing: v0.8.0

## What version of Knative Serving running on your cluster?

0.6.x

## Expected Behavior
When I run the command `kn service update XXX --image gcr.io/project/XXX` I would like that the latest version of my container is deployed. The latest version must be automatically checked against the GCR full checksum and not with the short name in the command.

## Actual Behavior
Nothing is performed because the kn tool check only the name of the image and not the tag of the latest version in the GCR.

## Steps to Reproduce the Problem

* Run `kn service create XXX --image gcr.io/project/XXX`
* Deploy a new version in GCR
* Run `kn service update XXX --image gcr.io/project/XXX`
-> No new revision created. But the container are really different.

## Workaround
I alternate the command

* `kn service update XXX --image gcr.io/project/XXX`
* `kn service update XXX --image gcr.io/project/XXX:latest`

Start at the `kn service update` command and trace how the client compares the requested image with the deployed image, then reproduce the case using the listed create, registry deployment, and update steps. Done means updating the same short image name after a new GCR image is available creates a new Knative Serving revision.

Written by the indexing model from the issue text.