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.