fast-import optimization:

fast-import optimization:

Now that cmd_data acts on a strbuf, make last_object stashed buffer be a
strbuf as well. On new stash, don't free the last stashed buffer, rather
swap it with the one you will stash, this way, callers of store_object can
act on static strbufs, and at some point, fast-import won't allocate new
memory for objects buffers.

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