git_config_maybe_bool()

git_config_maybe_bool()

Some configuration variables can take boolean values in addition to
enumeration specific to them.  Introduce git_config_maybe_bool() that
returns 0 or 1 if the given value is boolean, or -1 if not, so that
a parser for such a variable can check for boolean first and then
parse other kinds of values as a fallback.

Signed-off-by: Junio C Hamano <gitster@pobox.com>