themes: use `_bash_it_history_auto_save()`

First pass to use _Bash It_'s automatic history management.

themes/barbuk/barbuk.theme.bash

@@ -80,7 +80,7 @@ function _prompt {
 	_exit-code exit_code
 	_git-uptream-remote-logo
 
-	history -a
+	HISTCONTROL="${HISTCONTROL:-}:autosave" _bash_it_history_auto_save
 
 	# Detect root shell
 	if [ "$(whoami)" = root ]; then

themes/font/font.theme.bash

@@ -56,7 +56,7 @@ function prompt_command() {
     fi
 
     # Append new history lines to history file
-    history -a
+    HISTCONTROL="${HISTCONTROL:-}:autosave" _bash_it_history_auto_save
 
     PS1="$(clock_prompt)${virtualenv}$(user_host_prompt)${bold_cyan}\W $(scm_prompt_char_info)${ret_status}→ ${normal}"
 }