count-delta: tweak counting of copied source material.

count-delta: tweak counting of copied source material.

With the finer grained delta algorithm, count-delta algorithm
started overcounting copied source material, since the new delta
output tends to reuse the same source range more than once and
more aggressively.  This broke an earlier assumption that the
number of bytes copied out from the source buffer is a good
approximation how much source material is actually remaining in
the result.

This uses fairly inefficient algorithm to keep track of ranges
of source material that are actually copied out to the
destination buffer.  With this tweak, the obvious rename/break
detection tests in the testsuite start to work again.

Signed-off-by: Junio C Hamano <junkio@cox.net>