git_config_set: do not use a state machine

git_config_set: do not use a state machine

While a neat theoretical construct, state machines are hard to read. In
this instance, it does not even make a whole lot of sense because we are
more interested in flags, anyway: has the section been seen? Has the key
been seen? Does the current section match the key we are looking for?

Besides, the state `SECTION_SEEN` was named in a misleading way: it did
not indicate that we saw the section matching the key we are looking
for, but it instead indicated that we are *currently* in that section.

Let's just replace the state machine logic by clear and obvious flags.

This will also make it easier to review the upcoming patches to use the
newly-introduced `event_fn` callback of the config parser.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>