Reverse priority of tag lookup in TagStore.GetImage

Currently, if you have the following images:

    foo/bar      1       23b27d50fb49
    foo/bar      2       f2b86ec3fcc4

And you issue the following command:

    docker tag foo/bar:2 foo/bar latest

docker will tag the "wrong" image, because the image id for foo/bar:1 starts
with a "2". That is, you'll end up with the following:

    foo/bar      1       23b27d50fb49
    foo/bar      2       f2b86ec3fcc4
    foo/bar      latest  23b27d50fb49

This commit reverses the priority given to tags vs. image ids in the
construction `<user>/<repo>:<tagOrId>`, meaning that if a tag that is an exact
match for the specified `tagOrId`, it will be tagged in preference to an image
with an id that happens to start with the correct character sequence.