fast-import: treat cat-blob as a delta base hint for next blob

Delta base for blobs is chosen as a previously saved blob. If we
treat cat-blob's blob as a delta base for the next blob, nothing
is likely to become worse.

For fast-import stream producer like svn-fe cat-blob is used like
following:
- svn-fe reads file delta in svn format
- to apply it, svn-fe asks cat-blob 'svn delta base'
- applies 'svn delta' to the response
- produces a blob command to store the result

Currently there is no way for svn-fe to give fast-import a hint on
object delta base. While what's requested in cat-blob is most of
the time a best delta base possible. Of course, it could be not a
good delta base, but we don't know any better one anyway.

So do treat cat-blob's result as a delta base for next blob. The
profit is nice: 2x to 7x reduction in pack size AND 1.2x to 3x
time speedup due to diff_delta being faster on good deltas. git gc
--aggressive can compress it even more, by 10% to 70%, utilizing
more cpu time, real time and 3 cpu cores.

Tested on 213M and 2.7G fast-import streams, resulting packs are 22M
and 113M, import time is 7s and 60s, both streams are produced by
svn-fe, sniffed and then used as raw input for fast-import.

For git-fast-export produced streams there is no change as it doesn't
use cat-blob and doesn't try to reorder blobs in some smart way to
make successive deltas small.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
Acked-by: David Barr <davidbarr@google.com>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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