fetch: centralize printing of reference updates

fetch: centralize printing of reference updates

In order to print updated references during a fetch, the two different
call sites that do this will first call `format_display()` followed by a
call to `fputs()`. This is needlessly roundabout now that we have the
`display_state` structure that encapsulates all of the printing logic
for references.

Move displaying the reference updates into `format_display()` and rename
it to `display_ref_update()` to better match its new purpose, which
finalizes the conversion to make both the formatting and printing logic
of reference updates self-contained. This will make it easier to add new
output formats and printing to a different file descriptor than stderr.

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