ref-filter: move "cmp_fn" assignment into "else if" arm

ref-filter: move "cmp_fn" assignment into "else if" arm

Further amend code changed in 7c5045fc180 (ref-filter: apply fallback
refname sort only after all user sorts, 2020-05-03) to move an
assignment only used in the "else if" arm to happen there. Before that
commit the cmp_fn would be used outside of it.

We could also just skip the "cmp_fn" assignment and use
strcasecmp/strcmp directly in a ternary statement here, but this is
probably more readable.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>