Since we cd-ed into search dir in $cmd, prefix the $search_dir path
after fzf returns the results by using -filter switch of fzf.
Kakoune either needs an absolute path or path relative to its pwd
to edit a file. Since the pwd of $cmd and kakoune now differ,
we cannot use a relative path, so we construct the absolute path
by prefixing the $search_dir to the file outputted by fzf.
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