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>