receive-pack: use in_merge_bases() for fast-forward check

receive-pack: use in_merge_bases() for fast-forward check

The original computed merge-base between the old commit and the new
commit and checked if the old commit was a merge base between them,
in order to make sure we are fast-forwarding.

Instead, call in_merge_bases(old, new) which does the same.

Signed-off-by: Junio C Hamano <gitster@pobox.com>