Use a listfile to handle files import in case of large list
Fixed an issue that importer.php and exporter.php may shut up unexpectedly in large operation
Removed unused --work-dir option from exporter.php
Set up an empty working directory to prevent problems in bare repos
Prevent unnecessary I/O by sending loaded contents to the target file directly. Also fixed a potential fatal error with very large content.
Code style compliance: removes unnecessary spaces