fetch: move reference width calculation into `display_state`

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>