pack-bitmap-write: support storing pseudo-merge commits

pack-bitmap-write: support storing pseudo-merge commits

Prepare to write pseudo-merge bitmaps by annotating individual bitmapped
commits (which are represented by the `bitmapped_commit` structure) with
an extra bit indicating whether or not they are a pseudo-merge.

In subsequent commits, pseudo-merge bitmaps will be generated by
allocating a fake commit node with parents covering the full set of
commits represented by the pseudo-merge bitmap. These commits will be
added to the set of "selected" commits as usual, but will be written
specially instead of being included with the rest of the selected
commits.

Mechanically speaking, there are two parts of this change:

  - The bitmapped_commit struct gets a new bit indicating whether it is
    a pseudo-merge, or an ordinary commit selected for bitmaps.

  - A handful of changes to only write out the non-pseudo-merge commits
    when enumerating through the selected array (see the new
    `bitmap_writer_selected_nr()` function). Pseudo-merge commits appear
    after all non-pseudo-merge commits, so it is safe to enumerate
    through the selected array like so:

        for (i = 0; i < bitmap_writer_selected_nr(); i++)
          if (writer.selected[i].pseudo_merge)
            BUG("unexpected pseudo-merge");

    without encountering the BUG().

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