config: add options parameter to git_config_from_mem

config: add options parameter to git_config_from_mem

The underlying config parser knows how to handle a
config_options struct, but git_config_from_mem() always
passes NULL. Let's allow our callers to specify the options
struct.

We could add a "_with_options" variant, but since there are
only a handful of callers, let's just update them to pass
NULL.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>