pack-bitmap.c: read pseudo-merge extension

pack-bitmap.c: read pseudo-merge extension

Now that the scaffolding for reading the pseudo-merge extension has been
laid, teach the pack-bitmap machinery to read the pseudo-merge extension
when present.

Note that pseudo-merges themselves are not yet used during traversal,
this step will be taken by a future commit.

In the meantime, read the table and initialize the pseudo_merge_map
structure introduced by a previous commit. When the pseudo-merge
extension is present, `load_bitmap_header()` performs basic sanity
checks to make sure that the table is well-formed.

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