pack-bitmap-write.c: instrument number of reused bitmaps

pack-bitmap-write.c: instrument number of reused bitmaps

When debugging bitmap generation performance, it is useful to know how
many bitmaps were generated from scratch, and how many were the result
of permuting the bit-order of an existing bitmap.

Keep track of the latter, and emit the count as a trace2_data line to
aid in debugging.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>