Documentation/technical/bitmap-format.txt: add missing position table

While investigating a benign Coverity warning on the new pseudo-merge
implementation, I was struggling to understand the (paraphrased) below:

    ofs = index_end - 24 - (index->pseudo_merges.nr * sizeof(uint64_t));
    for (i = 0; i < index->pseudo_merges.nr; i++) {
            index->pseudo_merges.v[i].at = get_be64(ofs);
            ofs += sizeof(uint64_t);
    }

, in pack-bitmap.c::load_bitmap_header(). Looking at the documentation,
the diagram describing the on-disk format (prior to this patch)
suggested that the optional extended lookup table immediately preceded
the trailing metadata portion.

If that were the case, that would make the above code from
load_bitmap_header() incorrect, as we'd be blindly reading into the
extended offset table.

But later on in the documentation there is a description of the
pseudo-merge position table as immediately preceding the trailing
metadata portion of the extension. And indeed, we do write the position
table in pack-bitmap-write.c:

    /* write positions for all pseudo merges */
    for (i = 0; i < writer->pseudo_merges_nr; i++)
            hashwrite_be64(f, pseudo_merge_ofs[i]);

    hashwrite_be32(f, writer->pseudo_merges_nr);
    hashwrite_be32(f, kh_size(writer->pseudo_merge_commits));
    hashwrite_be64(f, table_start - start);
    hashwrite_be64(f, hashfile_total(f) - start + sizeof(uint64_t));

So this is purely a case of the diagram being out of sync with the
textual description and actual implementation of the format
specification.

Add the missing component back to the format diagram to avoid further
confusion in this area.

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

Documentation/technical/bitmap-format.txt

@@ -312,6 +312,15 @@ the end of a `.bitmap` file. The format is as follows:
 |                                           |
 +-------------------------------------------+
 |                                           |
+|  Pseudo-merge position table              |
+|  +----+----------+----------+----------+  |
+|  | N  | Offset 1 |   ....   | Offset N |  |
+|  +----+----------+----------+----------+  |
+|  |    |  8 bytes |   ....   |  8 bytes |  |
+|  +----+----------+----------+----------+  |
+|                                           |
++-------------------------------------------+
+|                                           |
 |  Pseudo-merge Metadata                    |
 |  +-----------------------------------+    |
 |  | # pseudo-merges (4 bytes)         |    |