feat(tvix/castore): add RenamedNode helper

This takes a castorev1pb.Node and updates its name, making sure to copy
it.

Change-Id: I57bf2b2b4a74496a1a697bcbce72bf67a12e69fe
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9594
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Brian McGee <brian@bmcgee.ie>

tvix/castore/protos/rename_node.go

@@ -0,0 +1,38 @@
+package castorev1
+
+// RenamedNode returns a node with a new name.
+func RenamedNode(node *Node, name string) *Node {
+	if directoryNode := node.GetDirectory(); directoryNode != nil {
+		return &Node{
+			Node: &Node_Directory{
+				Directory: &DirectoryNode{
+					Name:   []byte(name),
+					Digest: directoryNode.GetDigest(),
+					Size:   directoryNode.GetSize(),
+				},
+			},
+		}
+	} else if fileNode := node.GetFile(); fileNode != nil {
+		return &Node{
+			Node: &Node_File{
+				File: &FileNode{
+					Name:       []byte(name),
+					Digest:     fileNode.GetDigest(),
+					Size:       fileNode.GetSize(),
+					Executable: fileNode.GetExecutable(),
+				},
+			},
+		}
+	} else if symlinkNode := node.GetSymlink(); symlinkNode != nil {
+		return &Node{
+			Node: &Node_Symlink{
+				Symlink: &SymlinkNode{
+					Name:   []byte(name),
+					Target: symlinkNode.GetTarget(),
+				},
+			},
+		}
+	} else {
+		panic("unreachable")
+	}
+}