Fix successful return value for `SyncAndGetUserSpecificDiff` (#27152)

A function should not return an error when it is successful.
Otherwise, things like https://discord.com/channels/322538954119184384/322538954119184384/1153705341620600833 happen…

services/gitdiff/gitdiff.go

@@ -1343,7 +1343,7 @@ outer:
 		}
 	}
 
-	return diff, err
+	return diff, nil
 }
 
 // CommentAsDiff returns c.Patch as *Diff