pack-objects: implement bitmap writing

This commit extends more the functionality of `pack-objects` by allowing
it to write out a `.bitmap` index next to any written packs, together
with the `.idx` index that currently gets written.

If bitmap writing is enabled for a given repository (either by calling
`pack-objects` with the `--write-bitmap-index` flag or by having
`pack.writebitmaps` set to `true` in the config) and pack-objects is
writing a packfile that would normally be indexed (i.e. not piping to
stdout), we will attempt to write the corresponding bitmap index for the
packfile.

Bitmap index writing happens after the packfile and its index has been
successfully written to disk (`finish_tmp_packfile`). The process is
performed in several steps:

    1. `bitmap_writer_set_checksum`: this call stores the partial
       checksum for the packfile being written; the checksum will be
       written in the resulting bitmap index to verify its integrity

    2. `bitmap_writer_build_type_index`: this call uses the array of
       `struct object_entry` that has just been sorted when writing out
       the actual packfile index to disk to generate 4 type-index bitmaps
       (one for each object type).

       These bitmaps have their nth bit set if the given object is of
       the bitmap's type. E.g. the nth bit of the Commits bitmap will be
       1 if the nth object in the packfile index is a commit.

       This is a very cheap operation because the bitmap writing code has
       access to the metadata stored in the `struct object_entry` array,
       and hence the real type for each object in the packfile.

    3. `bitmap_writer_reuse_bitmaps`: if there exists an existing bitmap
       index for one of the packfiles we're trying to repack, this call
       will efficiently rebuild the existing bitmaps so they can be
       reused on the new index. All the existing bitmaps will be stored
       in a `reuse` hash table, and the commit selection phase will
       prioritize these when selecting, as they can be written directly
       to the new index without having to perform a revision walk to
       fill the bitmap. This can greatly speed up the repack of a
       repository that already has bitmaps.

    4. `bitmap_writer_select_commits`: if bitmap writing is enabled for
       a given `pack-objects` run, the sequence of commits generated
       during the Counting Objects phase will be stored in an array.

       We then use that array to build up the list of selected commits.
       Writing a bitmap in the index for each object in the repository
       would be cost-prohibitive, so we use a simple heuristic to pick
       the commits that will be indexed with bitmaps.

       The current heuristics are a simplified version of JGit's
       original implementation. We select a higher density of commits
       depending on their age: the 100 most recent commits are always
       selected, after that we pick 1 commit of each 100, and the gap
       increases as the commits grow older. On top of that, we make sure
       that every single branch that has not been merged (all the tips
       that would be required from a clone) gets their own bitmap, and
       when selecting commits between a gap, we tend to prioritize the
       commit with the most parents.

       Do note that there is no right/wrong way to perform commit
       selection; different selection algorithms will result in
       different commits being selected, but there's no such thing as
       "missing a commit". The bitmap walker algorithm implemented in
       `prepare_bitmap_walk` is able to adapt to missing bitmaps by
       performing manual walks that complete the bitmap: the ideal
       selection algorithm, however, would select the commits that are
       more likely to be used as roots for a walk in the future (e.g.
       the tips of each branch, and so on) to ensure a bitmap for them
       is always available.

    5. `bitmap_writer_build`: this is the computationally expensive part
       of bitmap generation. Based on the list of commits that were
       selected in the previous step, we perform several incremental
       walks to generate the bitmap for each commit.

       The walks begin from the oldest commit, and are built up
       incrementally for each branch. E.g. consider this dag where A, B,
       C, D, E, F are the selected commits, and a, b, c, e are a chunk
       of simplified history that will not receive bitmaps.

            A---a---B--b--C--c--D
                     \
                      E--e--F

       We start by building the bitmap for A, using A as the root for a
       revision walk and marking all the objects that are reachable
       until the walk is over. Once this bitmap is stored, we reuse the
       bitmap walker to perform the walk for B, assuming that once we
       reach A again, the walk will be terminated because A has already
       been SEEN on the previous walk.

       This process is repeated for C, and D, but when we try to
       generate the bitmaps for E, we can reuse neither the current walk
       nor the bitmap we have generated so far.

       What we do now is resetting both the walk and clearing the
       bitmap, and performing the walk from scratch using E as the
       origin. This new walk, however, does not need to be completed.
       Once we hit B, we can lookup the bitmap we have already stored
       for that commit and OR it with the existing bitmap we've composed
       so far, allowing us to limit the walk early.

       After all the bitmaps have been generated, another iteration
       through the list of commits is performed to find the best XOR
       offsets for compression before writing them to disk. Because of
       the incremental nature of these bitmaps, XORing one of them with
       its predecesor results in a minimal "bitmap delta" most of the
       time. We can write this delta to the on-disk bitmap index, and
       then re-compose the original bitmaps by XORing them again when
       loaded.

       This is a phase very similar to pack-object's `find_delta` (using
       bitmaps instead of objects, of course), except the heuristics
       have been greatly simplified: we only check the 10 bitmaps before
       any given one to find best compressing one. This gives good
       results in practice, because there is locality in the ordering of
       the objects (and therefore bitmaps) in the packfile.

     6. `bitmap_writer_finish`: the last step in the process is
	serializing to disk all the bitmap data that has been generated
	in the two previous steps.

	The bitmap is written to a tmp file and then moved atomically to
	its final destination, using the same process as
	`pack-write.c:write_idx_file`.

Signed-off-by: Vicent Marti <tanoku@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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