builtin/config: convert `do_not_match` to a local variable

builtin/config: convert `do_not_match` to a local variable

The `do_not_match` variable is used by the `format_config()` callback as
an indicator whether or not the passed regular expression is negated. It
is only ever set up by its only caller, `collect_config()` and can thus
easily be moved into the `collect_config_data` structure.

Do so to remove our reliance on global state.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>