midx.c: instrument MIDX and bitmap generation with trace2 regions

midx.c: instrument MIDX and bitmap generation with trace2 regions

When debugging MIDX and MIDX-bitmap related issues, it is useful to
figure out where Git is spending its time.

GitHub has been using the below trace2 regions to instrument various
components of generating a MIDX itself, as well time spent preparing to
build a MIDX bitmap.

These are limited to instrumenting the following functions:

  - midx.c::find_commits_for_midx_bitmap()
  - midx.c::midx_pack_order()
  - midx.c::prepare_midx_packing_data()
  - midx.c::write_midx_bitmap()
  - midx.c::write_midx_internal()
  - midx.c::write_midx_reverse_index()

to start and end with a trace2_region_enter() and trace2_region_leave(),
respectively.

The category for all of these is "midx", which matches the existing
convention. The region description matches the name of the function.

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