Error when gh repo rename is used with a new repo name that contains an owner

`gh repo rename` can only rename a repo, but not change its owner
(i.e. transfer it).

As flagged in #10034, at the moment, the CLI behaves like it can do
this, and produces weird results.

If I ask to rename my repo to `polyseam/frappe-containers`:

```bash
gh repo rename polyseam/frappe-containers
```

..the preview suggests that it will do the right thing:

```
? Rename polyseam/containers to polyseam/frappe-containers? Yes
```

...but the repo gets renamed wrongly:

```
✓ Renamed repository polyseam/polyseam-frappe-containers
```

This adds an upfront validation that looks for a slash and tells
you that the command can't transfer a repo.

Fixes #10034.

acceptance/testdata/repo/repo-rename-transfer-ownership.txtar

@@ -0,0 +1,9 @@
+# Create a repository with a file so it has a default branch
+exec gh repo create $ORG/$SCRIPT_NAME-$RANDOM_STRING --add-readme --private
+
+# Attempt to rename the repo with a slash in the name
+! exec gh repo rename $ORG/new-name --repo=$ORG/$SCRIPT_NAME-$RANDOM_STRING --yes
+stderr 'New repository name cannot contain '/' character - to transfer a repository to a new owner, you must follow additional steps on GitHub.com. For more information on transferring repository ownership, see <https://docs.github.com/en/repositories/creating-and-managing-repositories/transferring-a-repository>.'
+
+# Defer repo deletion
+defer gh repo delete $ORG/$SCRIPT_NAME-$RANDOM_STRING --yes

pkg/cmd/repo/rename/rename.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"net/http"
+	"strings"
 
 	"github.com/MakeNowJust/heredoc"
 	"github.com/cli/cli/v2/api"
@@ -58,7 +59,7 @@ func NewCmdRename(f *cmdutil.Factory, runf func(*RenameOptions) error) *cobra.Co
 			with %[1]s--repo%[1]s is renamed.
 			
 			To transfer repository ownership to another user account or organization,
-			you must follow additional steps on GitHub.com
+			you must follow additional steps on GitHub.com.
 
 			For more information on transferring repository ownership, see:
 			<https://docs.github.com/en/repositories/creating-and-managing-repositories/transferring-a-repository>
@@ -124,6 +125,10 @@ func renameRun(opts *RenameOptions) error {
 		}
 	}
 
+	if strings.Contains(newRepoName, "/") {
+		return fmt.Errorf("New repository name cannot contain '/' character - to transfer a repository to a new owner, you must follow additional steps on GitHub.com. For more information on transferring repository ownership, see <https://docs.github.com/en/repositories/creating-and-managing-repositories/transferring-a-repository>.")
+	}
+
 	if opts.DoConfirm {
 		var confirmed bool
 		if confirmed, err = opts.Prompter.Confirm(fmt.Sprintf(

pkg/cmd/repo/rename/rename_test.go

@@ -113,6 +113,8 @@ func TestRenameRun(t *testing.T) {
 		promptStubs func(*prompter.MockPrompter)
 		wantOut     string
 		tty         bool
+		wantErr     bool
+		errMsg      string
 	}{
 		{
 			name:    "none argument",
@@ -217,6 +219,16 @@ func TestRenameRun(t *testing.T) {
 			},
 			wantOut: "",
 		},
+
+		{
+			name: "error on name with slash",
+			tty:  true,
+			opts: RenameOptions{
+				newRepoSelector: "org/new-name",
+			},
+			wantErr: true,
+			errMsg:  "New repository name cannot contain '/' character - to transfer a repository to a new owner, you must follow additional steps on GitHub.com. For more information on transferring repository ownership, see <https://docs.github.com/en/repositories/creating-and-managing-repositories/transferring-a-repository>.",
+		},
 	}
 
 	for _, tt := range testCases {
@@ -268,6 +280,10 @@ func TestRenameRun(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			defer reg.Verify(t)
 			err := renameRun(&tt.opts)
+			if tt.wantErr {
+				assert.EqualError(t, err, tt.errMsg)
+				return
+			}
 			assert.NoError(t, err)
 			assert.Equal(t, tt.wantOut, stdout.String())
 		})