Allow single character repository names

The main goal of this changeset is to allow repository name components to
consist of a single character. The number of components allowed and the slash
separation requirements have also been clarified.

To go along with this simplification, errant constants and unneeded error types
have been removed.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

registry/api/v2/names_test.go

@@ -1,6 +1,7 @@
 package v2
 
 import (
+	"strconv"
 	"strings"
 	"testing"
 )
@@ -10,6 +11,10 @@ func TestRepositoryNameRegexp(t *testing.T) {
 		input string
 		err   error
 	}{
+		{
+			input: "",
+			err:   ErrRepositoryNameEmpty,
+		},
 		{
 			input: "short",
 		},
@@ -30,11 +35,26 @@ func TestRepositoryNameRegexp(t *testing.T) {
 		},
 		{
 			input: "a/a/a/b/b",
-			err:   ErrRepositoryNameComponentShort,
 		},
 		{
 			input: "a/a/a/a/",
-			err:   ErrRepositoryNameComponentShort,
+			err:   ErrRepositoryNameComponentInvalid,
+		},
+		{
+			input: "a//a/a",
+			err:   ErrRepositoryNameComponentInvalid,
+		},
+		{
+			input: "a",
+		},
+		{
+			input: "a/aa",
+		},
+		{
+			input: "aa/a",
+		},
+		{
+			input: "a/aa/a",
 		},
 		{
 			input: "foo.com/bar/baz",
@@ -58,10 +78,6 @@ func TestRepositoryNameRegexp(t *testing.T) {
 		{
 			input: "a-a/a-a",
 		},
-		{
-			input: "a",
-			err:   ErrRepositoryNameComponentShort,
-		},
 		{
 			input: "a-/a/a/a",
 			err:   ErrRepositoryNameComponentInvalid,
@@ -110,9 +126,8 @@ func TestRepositoryNameRegexp(t *testing.T) {
 			err:   ErrRepositoryNameComponentInvalid,
 		},
 	} {
-
 		failf := func(format string, v ...interface{}) {
-			t.Logf(testcase.input+": "+format, v...)
+			t.Logf(strconv.Quote(testcase.input)+": "+format, v...)
 			t.Fail()
 		}