config.c: rename git_config_maybe_bool_text and export it as git_parse_maybe_bool

This helper function does not complain about the config variable
but just silently reports failure to the caller.  It is useful for
callers that need to parse any string that could be boolean or other
string (e.g. tristate yes/no/auto).

Signed-off-by: Dave Borowitz <dborowitz@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>