write_entry: avoid reading blobs in CE_RETRY case

write_entry: avoid reading blobs in CE_RETRY case

When retrying a delayed filter-process request, we don't
need to send the blob to the filter a second time. However,
we read it unconditionally into a buffer, only to later
throw away that buffer. We can make this more efficient by
skipping the read in the first place when it isn't
necessary.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>