fix: regression for backslash-prefixed commands

Ref: issue #1973

shell/completion.zsh

@@ -126,7 +126,7 @@ __fzf_extract_command() {
   # Control completion with the "compstate" parameter, insert and list nothing
   compstate[insert]=
   compstate[list]=
-  cmd_word="${words[1]}"
+  cmd_word="${(Q)words[1]}"
 }
 
 __fzf_generic_path_completion() {