fetch: introduce `display_format` enum

fetch: introduce `display_format` enum

We currently have two different display formats in git-fetch(1) with the
"full" and "compact" formats. This is tracked with a boolean value that
simply denotes whether the display format is supposed to be compacted
or not. This works reasonably well while there are only two formats, but
we're about to introduce another format that will make this a bit more
awkward to use.

Introduce a `enum display_format` that is more readily extensible.

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