Provide more UI guidance when creating repositories

Summary: Ref T10923. Walk users through the "create, configure, activate" workflow a little better and set expectations more clearly.

Test Plan:
  - Created a new repository, saw new UI help.
  - Activated repository, saw onboarding help disappear.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10923

Differential Revision: https://secure.phabricator.com/D15875

src/applications/diffusion/controller/DiffusionRepositoryEditActivateController.php

@@ -47,7 +47,24 @@ final class DiffusionRepositoryEditActivateController
       $submit = pht('Deactivate Repository');
     } else {
       $title = pht('Activate Repository');
-      $body = pht('Activate this repository?');
+
+      $is_new = $repository->isNewlyInitialized();
+      if ($is_new) {
+        if ($repository->isHosted()) {
+          $body = pht(
+            'This repository will become a new hosted repository. '.
+            'It will begin serving read and write traffic.');
+        } else {
+          $body = pht(
+            'This repository will observe an existing remote repository. '.
+            'It will begin fetching changes from the remote.');
+        }
+      } else {
+        $body = pht(
+          'This repository will resume updates, observation, mirroring, '.
+          'and serving any configured read and write traffic.');
+      }
+
       $submit = pht('Activate Repository');
     }