config: teach git_config_set_multivar_in_file a default path

The git_config_set_multivar_in_file function takes a
filename argument to specify the file into which the values
should be written. Currently, this value must be non-NULL.
Callers which want to write to the default location must use
the regular, non-"in_file" version, which will either write
to config_exclusive_filename, or to the repo config if the
exclusive filename is NULL.

Let's migrate the "default to using repo config" logic into
the "in_file" form. That will let callers get the same
default-if-NULL behavior as one gets with
config_exclusive_filename, but without having to use the
global variable.

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

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