blame: drop blob data after passing blame to the parent

blame: drop blob data after passing blame to the parent

We used to keep the blob data for each origin that has any remaining
line in the result, but this will get very costly with a huge file that
has a deep history.  This patch releases the blob after we ran diff
between the child rev and its parents.  When passing blame from a parent
to its parent (i.e. the grandparent), the blob data for the parent may
need to be read again, but it should be relatively cheap, thanks to
delta-base cache.

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