plugin/history: no need to set a trap

Instead of globbally clearing `$HISTTIMEFORMAT` and setting a return trap to re-enable it, just make it local to the function.

Also, set the defaults in a way that is happy with read-only parameters.

# Conflicts:
#	plugins/available/history.plugin.bash

plugins/available/history.plugin.bash

@@ -2,26 +2,21 @@
 cite about-plugin
 about-plugin 'improve history handling with sane defaults'
 
-# append to bash_history if Terminal.app quits
+# append to bash_history on exit.
 shopt -s histappend
 
-# erase duplicates; alternative option: export HISTCONTROL=ignoredups
-export HISTCONTROL=${HISTCONTROL:-ignorespace:erasedups}
+# erase duplicates; alternative option: HISTCONTROL=ignoredups
+: "${HISTCONTROL:=ignorespace:erasedups}"
 
 # resize history to 100x the default (500)
-export HISTSIZE=${HISTSIZE:-50000}
+: "${HISTSIZE:=50000}"
 
 top-history() {
 	about 'print the name and count of the most commonly run tools'
 
-	if [[ -n $HISTTIMEFORMAT ]]; then
-		# To parse history we need a predictable format, which HISTTIMEFORMAT
-		# gets in the way of. So we unset it and set a trap to guarantee the
-		# user's environment returns to normal even if the pipeline below fails.
-		# shellcheck disable=SC2064
-		trap "export HISTTIMEFORMAT='$HISTTIMEFORMAT'" RETURN
-		unset HISTTIMEFORMAT
-	fi
+	# To parse history we need a predictable format, which HISTTIMEFORMAT
+	# gets in the way of. So we clear it locally.
+	local HISTTIMEFORMAT
 
 	history \
 		| awk '{