p5326: generate pack bitmaps before writing the MIDX bitmap

p5326: generate pack bitmaps before writing the MIDX bitmap

To help test the performance of permuting the contents of the hash-cache
when generating a MIDX bitmap, we need a bitmap which has its hash-cache
populated.

And since multi-pack bitmaps don't add *new* values to the hash-cache,
we have to rely on a single-pack bitmap to generate those values for us.

Therefore, generate a pack bitmap before the MIDX one in order to ensure
that the MIDX bitmap has entries in its hash-cache. Since we don't want
to time generating the pack bitmap, move that to a non-perf test run
before we try to generate the MIDX bitmap.

Likewise, get rid of the pack bitmap afterwords, to make certain that we
are not accidentally using it in the performance tests run later on.

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