[Search directory] fix preview when base dir contains spaces (patch 3)

There is still a bug afflicting the search directory feature. To reproduce, set -x fzf_preview_dir_cmd ls and search with a base directory with spaces in its name. The fix is to quote the entire command passed to eval

functions/__fzf_preview_file.fish

@@ -16,14 +16,14 @@ function __fzf_preview_file --description "Print a preview for the given file ba
     else if test -f "$file_path" # regular file
         if set --query fzf_preview_file_cmd
             # need to escape quotes to make sure eval receives file_path as a single arg
-            eval $fzf_preview_file_cmd \"$file_path\"
+            eval "$fzf_preview_file_cmd '$file_path'"
         else
             bat --style=numbers --color=always "$file_path"
         end
     else if test -d "$file_path" # directory
         if set --query fzf_preview_dir_cmd
             # see above
-            eval $fzf_preview_dir_cmd \"$file_path\"
+            eval "$fzf_preview_dir_cmd '$file_path'"
         else
             # -A list hidden files as well, except for . and ..
             # -F helps classify files by appending symbols after the file name