builtin-fsck: reports missing parent commits

builtin-fsck: reports missing parent commits

parse_commit ignores parent commits with certain errors
(eg. a non commit object is already loaded under the sha1 of
the parent). To make fsck reports such errors, it has to compare
the nummer of parent commits returned by parse commit with the
number of parent commits in the object or in the graft/shallow file.

Signed-off-by: Martin Koegler <mkoegler@auto.tuwien.ac.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>