docker save: fix the 'repositories' file

For various use cases, the 'repositories' file does not match expected
behavior.

Like,

	docker save busybox:latest | tar t

Before:

	[...]
	busybox:latest/
	busybox:latest/VERSION
	busybox:latest/json
	busybox:latest/layer.tar
	# note, the layer name, and lack of 'repositories' file

Now:

	[...]
	a9eb172552348a9a49180694790b33a1097f546456d041b6e82e4d7716ddb721/
	a9eb172552348a9a49180694790b33a1097f546456d041b6e82e4d7716ddb721/VERSION
	a9eb172552348a9a49180694790b33a1097f546456d041b6e82e4d7716ddb721/json
	a9eb172552348a9a49180694790b33a1097f546456d041b6e82e4d7716ddb721/layer.tar
	repositories
	# and the repositories file is correct for the single tagged
	# image.
	#> {"busybox":{"latest":"a9eb172552348a9a49180694790b33a1097f546456d041b6e82e4d7716ddb721"}}

and

	docker save a9eb17255234 | tar t

Before:

	[...]
	a9eb17255234/
	a9eb17255234/VERSION
	a9eb17255234/json
	a9eb17255234/layer.tar
	# Note the truncated layer name

Now:

	[...]
	a9eb172552348a9a49180694790b33a1097f546456d041b6e82e4d7716ddb721/
	a9eb172552348a9a49180694790b33a1097f546456d041b6e82e4d7716ddb721/VERSION
	a9eb172552348a9a49180694790b33a1097f546456d041b6e82e4d7716ddb721/json
	a9eb172552348a9a49180694790b33a1097f546456d041b6e82e4d7716ddb721/layer.tar
	# There is no 'repositories' file, because there is no named repo

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)