log.decorate: accept 0/1 bool values

We explicitly document "0" and "1" as synonyms for "false"
and "true" in boolean config options. However, we don't
actually handle those values in git_config_maybe_bool.

In most cases this works fine, as we call git_config_bool,
which in turn calls git_config_bool_or_int, which in turn
calls git_config_maybe_bool. Values of 0/1 are considered
"not bool", but their integer values end up being converted
to the corresponding boolean values.

However, the log.decorate code looks for maybe_bool
explicitly, so that it can fall back to the "short" and
"full" strings. It does not handle 0/1 at all, and considers
them invalid values.

We cannot simply add 0/1 support to git_config_maybe_bool.
That would confuse git_config_bool_or_int, which may want to
distinguish the integer values "0" and "1" from bools.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>