Support appending arbitrary lines to DisplayRef output

Summary:
Ref T13546. Several substeps in the new "arc land" flow benefit from this. For example:

  - When prompting "land revisions you don't own?", it's used to show authors.
  - When prompting "land revisions in the wrong state?", it's used to show the current states.

Test Plan: Ran future "arc land" workflows, got relevant contextual information via this mechanism.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21308

src/ref/ArcanistDisplayRef.php

@@ -7,6 +7,7 @@ final class ArcanistDisplayRef
 
   private $ref;
   private $uri;
+  private $lines = array();
 
   public function setRef(ArcanistRef $ref) {
     $this->ref = $ref;
@@ -26,6 +27,11 @@ final class ArcanistDisplayRef
     return $this->uri;
   }
 
+  public function appendLine($line) {
+    $this->lines[] = $line;
+    return $this;
+  }
+
   public function newTerminalString() {
     $ref = $this->getRef();
 
@@ -83,6 +89,10 @@ final class ArcanistDisplayRef
         $uri);
     }
 
+    foreach ($this->lines as $line) {
+      $output[] = tsprintf("        %s\n", $line);
+    }
+
     return $output;
   }