turn off history substitution in case it's been enabled (2), fixes #583

dash considers "set +H" is a syntax error and doesn't even evaluate the " || true" part
the workaround consists in trying "set +H" in a subshell first

.tmux.conf

@@ -178,7 +178,7 @@ run 'cut -c3- ~/.tmux.conf | sh -s _apply_configuration'
 #
 # unset GREP_OPTIONS
 # export LC_NUMERIC=C
-# set +H 2>/dev/null || true
+# (set +H 2>/dev/null) && set +H || true
 #
 # if ! printf '' | sed -E 's///' 2>/dev/null; then
 #   if printf '' | sed -r 's///' 2>/dev/null; then