commit: add function to unparse a commit and its parents

commit: add function to unparse a commit and its parents

This patch adds the "unparse_commit" function that returns a commit
into an unparsed state by freeing its data and resetting its fields
to 0.

Its parents are recursively unparsed too, because they might have
been changed. But its tree is not unparsed as it should not have
been modifed.

Note that as the "flags" and "used" fields may be used even if the
object is not parsed, we have to reset them anyway.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>