perf-test: add simple memory tests for notmuch-show

These are probably too slow to run with the full corpus

performance-test/M02-show.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+test_description='show'
+
+. ./perf-test-lib.sh || exit 1
+
+memory_start
+
+memory_run 'show *' "notmuch show '*' 1>/dev/null"
+memory_run 'show --format=json *' "notmuch show --format=json '*' 1>/dev/null"
+memory_run 'show --format=sexp *' "notmuch show --format=sexp '*' 1>/dev/null"
+
+memory_done