fsck: make fsck_commit() warn-friendly

fsck: make fsck_commit() warn-friendly

When fsck_commit() identifies a problem with the commit, it should try
to make it possible to continue checking the commit object, in case the
user wants to demote the detected errors to mere warnings.

Note that some problems are too problematic to simply ignore. For
example, when the header lines are mixed up, we punt after encountering
an incorrect line. Therefore, demoting certain warnings to errors can
hide other problems. Example: demoting the missingauthor error to
a warning would hide a problematic committer line.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>