When trying to use completion for mpv binaries specified with some shell
glob, e.g. ~/dev/mpv/build/mpv, the current code doesn't substitute the
homedir prefix into the path name, resulting in runtime errors about
the file '~/dev/mpv/build/mpv' not being found.
The simple fixed is to use $~var instead of $var whenever expanding the
filename, which performs the same globbing that would otherwise be
performed when executing the command.