PackWriter: Hoist and cluster reference targets

PackWriter: Hoist and cluster reference targets

Many source browsers and network related tools like UploadPack need
to find and parse the target of all branches and annotated tags
within the repository during their startup phase.  Clustering these
together into the same part of the pack file will improve locality,
reducing thrashing when an application starts and needs to load
all of these into memory at once.

To prevent bottlenecking basic log viewing tools that are scannning
backwards from the tip of a current branch (and don't need tags)
we place this cluster of older targets after 4096 newer commits
have already been placed into the pack stream.  4096 was chosen as
a rough guess, but was based on a few factors:

  - log viewers typically show 5-200 commits per page
  - users only view the first page or two

  - DHT can cram 2200-4000 commits per 1 MiB chunk
    thus these will fall into the second commit chunk (roughly)

Unfortunately this placement hurts history tools that are scanning
backwards through the commit graph and completely ignored tags or
branch heads when they started.

An ancient tagged commit is no longer positioned behind its first
child (its now much earlier), resulting in a page fault for the
parser to reload this cluster of objects on demand.  This may be
an acceptable loss.  If a user is walking backwards and has already
scanned through more than 4096 commits of history, waiting for the
region to reload isn't really that bad compared to the amount of
time already spent.

If the repository is so small that there are less than 4096 commits,
this change has no impact on the placement of objects.

Change-Id: If3052e430d305e17878d94145c93754f56b74c61
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>