largefiles: implement addremove (issue3064) Implementing addremove correctly in largefiles is tricky, becuase the original addremove function does not call into any of the add or remove function we've already overridden in the extension. So the trick is to implement addremove without duplicating any code. This patch implements addremove by pulling out the interesting parts of override_add() and override_remove() into generic utility functions, and using those to handle the largefiles in addremove. Then a matcher is installed that will ignore all largefiles, and the original addremove function is called to take care of the regular files in addremove. A small bit of monkey patching is used to make sure that remove_largefiles() notifies the user when a file is removed by addremove and also makes sure the removal of largefiles doesn't interfer with the original addremove's operation of removing the standin.