Documentation improvements and code cleanups for graph package

Expand the godoc documentation for the graph package.

Centralize DefaultTag in the graphs/tag package instead of defining it
twice.

Remove some unnecessary "config" structs that are only used to pass
a few parameters to a function.

Simplify the GetParentsSize function - there's no reason for it to take
an accumulator argument.

Unexport some functions that aren't needed outside the package.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

api/client/build.go

@@ -526,7 +526,7 @@ func rewriteDockerfileFrom(dockerfileName string, translator func(string, regist
 			// Replace the line with a resolved "FROM repo@digest"
 			repo, tag := parsers.ParseRepositoryTag(matches[1])
 			if tag == "" {
-				tag = tags.DEFAULTTAG
+				tag = tags.DefaultTag
 			}
 			ref := registry.ParseReference(tag)
 

api/client/create.go

@@ -26,7 +26,7 @@ func (cli *DockerCli) pullImageCustomOut(image string, out io.Writer) error {
 	repos, tag := parsers.ParseRepositoryTag(image)
 	// pull only the image tagged 'latest' if no tag was specified
 	if tag == "" {
-		tag = tags.DEFAULTTAG
+		tag = tags.DefaultTag
 	}
 	v.Set("fromImage", repos)
 	v.Set("tag", tag)
@@ -96,7 +96,7 @@ func (cli *DockerCli) createContainer(config *runconfig.Config, hostConfig *runc
 
 	repo, tag := parsers.ParseRepositoryTag(config.Image)
 	if tag == "" {
-		tag = tags.DEFAULTTAG
+		tag = tags.DefaultTag
 	}
 
 	ref := registry.ParseReference(tag)

api/client/pull.go

@@ -25,7 +25,7 @@ func (cli *DockerCli) CmdPull(args ...string) error {
 
 	taglessRemote, tag := parsers.ParseRepositoryTag(remote)
 	if tag == "" && !*allTags {
-		tag = tags.DEFAULTTAG
+		tag = tags.DefaultTag
 		fmt.Fprintf(cli.out, "Using default tag: %s\n", tag)
 	} else if tag != "" && *allTags {
 		return fmt.Errorf("tag can't be used with --all-tags/-a")