fetch: move reference width calculation into `display_state`
In order to print references in proper columns we need to calculate the
width of the reference column before starting to print the references.
This is done with the help of a global variable `refcol_width`.
Refactor the code to instead use a new structure `display_state` that
contains the computed width and plumb it through the stack as required.
This is only the first step towards de-globalizing the state required to
print references.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>