blame.c: don't drop origin blobs as eagerly

blame.c: don't drop origin blobs as eagerly

When a parent blob already has chunks queued up for blaming, dropping
the blob at the end of one blame step will cause it to get reloaded
right away, doubling the amount of I/O and unpacking when processing a
linear history.

Keeping such parent blobs in memory seems like a reasonable optimization
that should incur additional memory pressure mostly when processing the
merges from old branches.

Signed-off-by: David Kastrup <redacted>
Signed-off-by: Junio C Hamano <redacted>