list-objects: don't queue root trees unless revs->tree_objects is set

list-objects: don't queue root trees unless revs->tree_objects is set

When traverse_commit_list() processes each commit, it queues the
commit's root tree in the pending array. Then, after all commits are
processed, it calls traverse_trees_and_blobs() to walk over the pending
list, calling process_tree() on each. But if revs->tree_objects is not
set, process_tree() just exists immediately!

We can save ourselves some work by not even bothering to queue these
trees in the first place. There are a few subtle points to make:

  - we also detect commits with a NULL tree pointer here. But this isn't
    an interesting check for broken commits, since the lookup_tree()
    we'd have done during commit parsing doesn't actually check that we
    have the tree on disk. So we're not losing any robustness.

  - besides queueing, we also set the NOT_USER_GIVEN flag on the tree
    object. This is used by the traverse_commit_list_filtered() variant.
    But if we're not exploring trees, then we won't actually care about
    this flag, which is used only inside process_tree() code-paths.

  - queueing trees eventually leads to us queueing blobs, too. But we
    don't need to check revs->blob_objects here. Even in the current
    code, we still wouldn't find those blobs, because we'd never open up
    the tree objects to list their contents.

  - the user-visible impact to the caller is minimal. The pending trees
    are all cleared by the time the function returns anyway, by
    traverse_trees_and_blobs(). We do call a show_commit() callback,
    which technically could be looking at revs->pending during the
    callback. But it seems like a rather unlikely thing to do (if you
    want the tree of the current commit, then accessing the tree struct
    member is a lot simpler).

So this should be safe to do. Let's look at the benefits:

  [before]
  Benchmark #1: git -C linux rev-list HEAD >/dev/null
    Time (mean ± σ):      7.651 s ±  0.021 s    [User: 7.399 s, System: 0.252 s]
    Range (min … max):    7.607 s …  7.683 s    10 runs

  [after]
  Benchmark #1: git -C linux rev-list HEAD >/dev/null
    Time (mean ± σ):      7.593 s ±  0.023 s    [User: 7.329 s, System: 0.264 s]
    Range (min … max):    7.565 s …  7.634 s    10 runs

Not too impressive, but then we're really just avoiding sticking a
pointer into a growable array. But still, I'll take a free 0.75%
speedup.

Let's try it after running "git commit-graph write":

  [before]
  Benchmark #1: git -C linux rev-list HEAD >/dev/null
    Time (mean ± σ):      1.458 s ±  0.011 s    [User: 1.199 s, System: 0.259 s]
    Range (min … max):    1.447 s …  1.481 s    10 runs

  [after]
  Benchmark #1: git -C linux rev-list HEAD >/dev/null
    Time (mean ± σ):      1.126 s ±  0.023 s    [User: 896.5 ms, System: 229.0 ms]
    Range (min … max):    1.106 s …  1.181 s    10 runs

Now that's more like it. We saved over 22% of the total time. Part of
that is because the runtime is shorter overall, but the absolute
improvement is also much larger. What's going on?

When we fill in a commit struct using the commit graph, we don't bother
to set the tree pointer, and instead lazy-load it when somebody calls
get_commit_tree(). So we're not only skipping the pointer write to the
pending queue, but we're skipping the lazy-load of the tree entirely.

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