docs(application): add comments explaining commit selection logic for rollback support

Add clarifying comments to the setGitImportSettings method explaining how the
commit selection works, including the fallback to git_commit_sha and that invalid
refs will cause failures on the remote server. This documents the behavior
introduced for proper rollback commit handling.

Also remove an extra blank line for minor code cleanup.

app/Models/Application.php

@@ -1093,6 +1093,8 @@ class Application extends BaseModel
         $escapedBaseDir = escapeshellarg($baseDir);
         $isShallowCloneEnabled = $this->settings?->is_git_shallow_clone_enabled ?? false;
 
+        // Use the explicitly passed commit (e.g. from rollback), falling back to the application's git_commit_sha.
+        // Invalid refs will cause the git checkout/fetch command to fail on the remote server.
         $commitToUse = $commit ?? $this->git_commit_sha;
 
         if ($commitToUse !== 'HEAD') {
@@ -1964,7 +1966,6 @@ class Application extends BaseModel
         }
     }
 
-
     public function getLimits(): array
     {
         return [