Make it possible to disable Ctrl+T / Alt+C / completions (#3678)

This makes it possible to skip one of the above key bindings or
completions by setting a variable to an empty string. For example,

    FZF_CTRL_T_COMMAND= FZF_ALT_C_COMMAND= \
      eval "$(fzf --zsh)"

Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

test/test_go.rb

@@ -18,7 +18,6 @@ UNSETS = %w[
   FZF_ALT_C_COMMAND
   FZF_ALT_C_OPTS FZF_CTRL_R_OPTS
   FZF_API_KEY
-  fish_history
 ].freeze
 DEFAULT_TIMEOUT = 10
 
@@ -67,7 +66,7 @@ class Shell
     end
 
     def fish
-      UNSETS.map { |v| v + '= ' }.join + ' FZF_DEFAULT_OPTS=--no-scrollbar fish'
+      "unset #{UNSETS.join(' ')}; FZF_DEFAULT_OPTS=--no-scrollbar fish_history= fish"
     end
   end
 end