Switch file uploader in "arc diff" to use ConduitEngine

Summary: Fixes T13498. Currently, `arc diff` may invoke the file upload flow using an older variation of the API. Both the modern and fallback clients build an Engine, so switching to the modern API is trivial.

Test Plan:
  - Created a local commit with a 1MB binary file.
  - Ran `arc diff --only`.
  - Before patch: fatal on old API call.
  - After patch: clean upload.

Maniphest Tasks: T13498

Differential Revision: https://secure.phabricator.com/D21043

src/upload/ArcanistFileUploader.php

@@ -5,7 +5,7 @@
  *
  *   // Create a new uploader.
  *   $uploader = id(new ArcanistFileUploader())
- *     ->setConduitClient($conduit);
+ *     ->setConduitEngine($conduit);
  *
  *   // Queue one or more files to be uploaded.
  *   $file = id(new ArcanistFileDataRef())
@@ -77,7 +77,7 @@ final class ArcanistFileUploader extends Phobject {
    * Upload files to the server.
    *
    * This transfers all files which have been queued with @{method:addFiles}
-   * over the Conduit link configured with @{method:setConduitClient}.
+   * over the Conduit link configured with @{method:setConduitEngine}.
    *
    * This method returns a map of all file data references. If references were
    * added with an explicit key when @{method:addFile} was called, the key is

src/workflow/ArcanistDiffWorkflow.php

@@ -2742,7 +2742,7 @@ EOTEXT
     }
 
     $uploader = id(new ArcanistFileUploader())
-      ->setConduitClient($this->getConduit());
+      ->setConduitEngine($this->getConduitEngine());
 
     foreach ($need_upload as $key => $spec) {
       $ref = id(new ArcanistFileDataRef())