bisect: Fix log output for multi-parent skip ranges

bisect: Fix log output for multi-parent skip ranges

The bisect log output of skipped commits introduced in f989cac "bisect:
Log possibly bad, skipped commits at bisection end" should obtain the range of
skipped commits from

    git rev-list bad --not good-1 good-2

not

    git rev-list bad --not good-1 --not good-2

when the skipped range contains a merge with good points in each parent.

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