pacdiff: Add new backup option to fish wrapper function
pacdiff: Add new backup option to fish wrapper function

The `--backup` option is introduced by pacdiff 1.6.0 (see
https://gitlab.archlinux.org/pacman/pacman-contrib/-/blob/35e1e7f4730cd9b791fbbc88b3f57b83f911d0cb/CHANGES.md#anchor-160).
Enabling it by default prevents the loss of information (specifically,
of the contents of the old file) that previously occurred when executing
the "Overwrite with pacnew" action.

razzolini/dotfiles,razzolini/dotfiles,razzolini/dotfiles