Avoid loading commits twice in log with diffs

Avoid loading commits twice in log with diffs

If you run a log with diffs (such as -p, --raw, --stat etc.) the
current code ends up loading many objects twice.  For example, for
'log -3000 -p' my instrumentation said the objects loaded more than
once are distributed as follows:

  2008 blob
  2103 commit
  2678 tree

Fixing blobs and trees will be harder, because those are really used
within the diff engine and need some form of caching.

However, fixing the commits is easy at least at the band-aid level.
They are triggered by log_tree_diff() invoking diff_tree_sha1() on
commits, which duly loads the specified object to dereference it to a
tree.  Since log_tree_diff() knows that it works with commits and they
must have trees, we can simply pass through the trees.

We add some parse_commit() calls.  The ones for the parents are
required; we do not know at this stage if they have been looked at.
The one for the commit itself is pure paranoia, but has about the same
cost as an assertion on commit->object.parsed.

This has a quite dramatic effect on log --raw, though only a
negligible impact on log -p:

Test                      this tree         HEAD
--------------------------------------------------------------------
4000.2: log --raw -3000   0.50(0.43+0.06)   0.54(0.46+0.06) +7.0%***
4000.3: log -p -3000      2.34(2.20+0.13)   2.37(2.22+0.13) +1.2%
--------------------------------------------------------------------
Significance hints:  '.' 0.1  '*' 0.05  '**' 0.01  '***' 0.001

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>