Hand the preview address to Vim as an argument again

Through the environment it depended on WSLENV reaching a Windows vim.exe, and
the guard for that turned a missing address into a blank preview. An argument
always arrives. Doubling the quotes keeps it one ':execute' string, so the
sandbox still covers all of it.

bin/tagpreview.sh

@@ -32,22 +32,15 @@ fi
 
 # The address comes from a tags file, which can be untrusted, and a preview runs
 # as soon as an entry is highlighted. Sandboxed as a builtin tag jump is, so that
-# it cannot run a shell command or touch a file. ':sandbox' on its own only
-# covers the command up to the first '|', hence 'sandbox execute' on a value
-# passed through the environment, which keeps the whole address inside
-export FZFVIM_EXCMD="${EXCMD}"
-# WSL shares only the variables named in WSLENV with a Windows vim.exe
-export WSLENV="FZFVIM_EXCMD/w${WSLENV:+:${WSLENV}}"
-# Ex mode starts on the last line, so an address that did not reach Vim would
-# center the preview there rather than say anything. Show nothing instead
-REQUIRE_EXCMD=""
-if [ -n "${EXCMD}" ]; then
-  REQUIRE_EXCMD='if empty($FZFVIM_EXCMD) | cquit | endif'
-fi
+# it cannot run a shell command or touch a file. ':sandbox' on its own only covers
+# the command up to the first '|', so hand the whole address to ':execute' as a
+# string literal, doubling the quotes in it. A newline would end that line and
+# leave the rest outside the sandbox, and no tags file holds one, so drop them
+EXCMD=${EXCMD//$'\n'/ }
+EXCMD_LITERAL=${EXCMD//\'/\'\'}
 CENTER="$("${VIMNAME}" -R -i NONE -u NONE -e -m -s "${FILE}" \
               -c "set nomagic" \
-              -c "${REQUIRE_EXCMD}" \
-              -c 'silent sandbox execute $FZFVIM_EXCMD' \
+              -c "silent sandbox execute '${EXCMD_LITERAL}'" \
               -c 'let l=line(".") | new | put =l | print | qa!')" || exit
 
 START_LINE="$(( CENTER - FZF_PREVIEW_LINES / 2 ))"