ZSH_FZF_HISTORY_SEARCH_FZF_ARGS and
ZSH_FZF_HISTORY_SEARCH_FZF_EXTRA_ARGS may contain multiple arguments
separated by spaces that need to be expanded to build the command line
for fzf. This is currently done by calling a subshell and letting
echo handle it.
Unlike other shells, zsh does not do word splitting by default.
According to zshexpn(1) you can perform word splitting using the rules
for SH_WORD_SPLIT by using the ${=spec} construct. Avoid creating a
subshell and just expand the spaces.
