describe: pass commit to describe_commit()

describe: pass commit to describe_commit()

There's a call in describe_commit() to lookup_commit_reference(), but we
don't check the return value. If it returns NULL, we'll segfault as we
immediately dereference the result.

In practice this can never happen, since all callers pass an oid which
came from a "struct commit" already. So we can make this more obvious
by just taking that commit struct in the first place.

Reported-by: Cheng <prophecheng@stu.pku.edu.cn>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>