lib/history: new functions _bash-it-history-auto-*()

Two new functions `_bash-it-history-auto-save()` and `_bash-it-history-auto-load()`, which append new history to disk and load new history from disk, respectively.

See bash-it/bash-it#1595 for discussion.

lib/history.bash

@@ -0,0 +1,49 @@
+# shellcheck shell=bash
+#
+# Functions for working with Bash's command history.
+
+function _bash-it-history-init() {
+	safe_append_preexec '_bash-it-history-auto-save'
+	safe_append_prompt_command '_bash-it-history-auto-load'
+}
+
+function _bash-it-history-auto-save() {
+	case $HISTCONTROL in
+		*'noauto'* | *'autoload'*)
+			: # Do nothing, as configured.
+			;;
+		*'auto'*)
+			# Append new history from this session to the $HISTFILE
+			history -a
+			;;
+		*)
+			# Append *only* if shell option `histappend` has been enabled.
+			shopt -q histappend && history -a && return
+			;;
+	esac
+}
+
+function _bash-it-history-auto-load() {
+	case $HISTCONTROL in
+		*'noauto'*)
+			: # Do nothing, as configured.
+			;;
+		*'autosave'*)
+			# Append new history from this session to the $HISTFILE
+			history -a
+			;;
+		*'autoloadnew'*)
+			# Read new entries from $HISTFILE
+			history -n
+			;;
+		*'auto'*)
+			# Blank in-memory history, then read entire $HISTFILE fresh from disk.
+			history -a && history -c && history -r
+			;;
+		*)
+			: # Do nothing, default.
+			;;
+	esac
+}
+
+_bash_it_library_finalize_hook+=('_bash-it-history-init')