[FileBackend] Set ignore_user_abort() in file operations.

* This reduces the change of partial operations. WMF sites already
  set ignore_user_abort() in configuration, but this makes sure that
  it always happens during file changes.

Change-Id: I702c27fc3c19aca0cdd39b793a3250ead40bfe71
Merge "[FileBackend] Set ignore_user_abort() in file operations."