config: don't have setters depend on `the_repository`

config: don't have setters depend on `the_repository`

Some of the setters that accept a `struct repository` still implicitly
rely on `the_repository` via `git_config_set_multivar_in_file()`. While
this function would typically use the caller-provided path, it knows to
fall back to using the configuration path indicated by `the_repository`.

Adapt those functions to instead use the caller-provided repository.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>