fetch: pass through `fetch_config` directly

fetch: pass through `fetch_config` directly

The `fetch_config` structure currently only has a single member, which
is the `display_format`. We're about extend it to contain all parsed
config values and will thus need it available in most of the code.

Prepare for this change by passing through the `fetch_config` directly
instead of only passing its single member.

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