[completion] Use file redirection instead of pipe

This change allows the completion system of bash and zsh to return
before the input process completes.

Related #1887

shell/completion.bash

@@ -200,7 +200,7 @@ _fzf_complete() {
   if [[ "$cur" == *"$trigger" ]]; then
     cur=${cur:0:${#cur}-${#trigger}}
 
-    selected=$(cat | FZF_DEFAULT_OPTS="--height ${FZF_TMUX_HEIGHT:-40%} --reverse $FZF_DEFAULT_OPTS $FZF_COMPLETION_OPTS $1" __fzf_comprun "$2" -q "$cur" | $post | tr '\n' ' ')
+    selected=$(FZF_DEFAULT_OPTS="--height ${FZF_TMUX_HEIGHT:-40%} --reverse $FZF_DEFAULT_OPTS $FZF_COMPLETION_OPTS $1" __fzf_comprun "$2" -q "$cur" | $post | tr '\n' ' ')
     selected=${selected% } # Strip trailing space not to repeat "-o nospace"
     if [ -n "$selected" ]; then
       COMPREPLY=("$selected")

shell/completion.zsh

@@ -116,7 +116,7 @@ _fzf_complete() {
   type $post > /dev/null 2>&1 || post=cat
 
   _fzf_feed_fifo "$fifo"
-  matches=$(cat "$fifo" | FZF_DEFAULT_OPTS="--height ${FZF_TMUX_HEIGHT:-40%} --reverse $FZF_DEFAULT_OPTS $FZF_COMPLETION_OPTS" __fzf_comprun "$cmd" ${(Q)${(Z+n+)fzf_opts}} -q "${(Q)prefix}" | $post | tr '\n' ' ')
+  matches=$(FZF_DEFAULT_OPTS="--height ${FZF_TMUX_HEIGHT:-40%} --reverse $FZF_DEFAULT_OPTS $FZF_COMPLETION_OPTS" __fzf_comprun "$cmd" ${(Q)${(Z+n+)fzf_opts}} -q "${(Q)prefix}" < "$fifo" | $post | tr '\n' ' ')
   if [ -n "$matches" ]; then
     LBUFFER="$lbuf$matches"
   fi