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

Author: gaelicWizard

plugins/available/history.plugin.bash

@@ -5,23 +5,26 @@ about-plugin 'improve history handling with sane defaults'
 # variable when the shell exits, rather than overwriting the file.
 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}"
 
 # Flush history to disk after each command.
 export PROMPT_COMMAND="history -a;${PROMPT_COMMAND}"
 
 function top-history() {
 	about 'print the name and count of the most commonly run tools'
+	# To parse history we need a predictable format, which HISTTIMEFORMAT
+	# gets in the way of. So we clear it locally.
+	local HISTTIMEFORMAT
 
 	# - Make sure formatting doesn't interfer with our parsing
 	# - Use awk to count how many times the first command on each line has been called
 	# - Truncate to 10 lines
 	# - Print in column format
-	HISTTIMEFORMAT='' history \
+	history \
 		| awk '{
 				a[$2]++
 			}END{