reflog expire: refactor & use "tip_commit" only for UE_NORMAL

reflog expire: refactor & use "tip_commit" only for UE_NORMAL

Add an intermediate variable for "tip_commit" in
reflog_expiry_prepare(), and only add it to the struct if we're
handling the UE_NORMAL case.

The code behaves the same way as before, but this makes the control
flow clearer, and the shorter name allows us to fold a 4-line i/else
into a one-line ternary instead.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>