/usr/share/vim/addons/plugin/editexisting.vim is in vim-runtime 2:7.4.052-1ubuntu3.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/vim/addons/plugin/../../vim74/macros/editexisting.vim
/usr/share/vim/addons/plugin/../../vim74/macros/editexisting.vim