When observing a repository, switch to "importing" mode on a large discovery in an empty repository

Summary:
Ref T10923. Fixes T9554.

When hosting a repository, we currently have a heuristic that tries to detect when you're doing an initial import: if you push more than 7 commits to an empty repository, it counts as an import and we disable mail/feed/etc.

Do something similar for observed repositories: if the repository is empty and we discover more than 7 commits, switch to import mode until we catch up.

This should align behavior with user expectation more often when juggling hosted vs imported repositories.

Test Plan:
  - Created a new hosted repository.
  - Activated it and allowed it to fully import.
  - Added an "Observe URI".
  - Saw it automatically drop into "Importing" mode until the import completed.
  - Swapped it back to hosted mode.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9554, T10923

Differential Revision: https://secure.phabricator.com/D15877

src/applications/diffusion/engine/DiffusionCommitHookEngine.php

@@ -172,14 +172,7 @@ final class DiffusionCommitHookEngine extends Phobject {
 
     if ($this->isInitialImport($all_updates)) {
       $repository = $this->getRepository();
-
-      $repository->openTransaction();
-        $repository->beginReadLocking();
-          $repository = $repository->reload();
-          $repository->setDetail('importing', true);
-          $repository->save();
-        $repository->endReadLocking();
-      $repository->saveTransaction();
+      $repository->markImporting();
     }
 
     if ($this->emailPHIDs) {
@@ -1244,7 +1237,7 @@ final class DiffusionCommitHookEngine extends Phobject {
       $commit_count++;
     }
 
-    if ($commit_count <= 7) {
+    if ($commit_count <= PhabricatorRepository::IMPORT_THRESHOLD) {
       // If this pushes a very small number of commits, assume it's an
       // initial commit or stack of a few initial commits.
       return false;