Lazy-load trees when reading commit-graph

There are several commit-graph walks that require loading many commits but never walk the trees reachable from those commits. However, the current logic in parse_commit() requires the root tree to be loaded. This only uses lookup_tree(), but when reading commits from the commit- graph file, the hashcpy() to load the root tree hash and the time spent checking the object cache take more time than parsing the rest of the commit.

In this patch series, all direct references to accessing the 'tree' member of struct commit are replaced instead by one of the following methods:

struct tree *get_commit_tree(struct commit *) struct object_id *get_commit_tree_oid(struct commit *)

This replacement was assisted by a Coccinelle script, but the 'tree' member is overloaded in other types, so we first rename the 'tree' member to 'maybe_tree' and use the compiler to ensure we caught all examples. Then, contrib/coccinelle/commit.cocci generates the patch to replace all accessors of 'maybe_tree' to the methods above.

After all access is restricted to use these methods, we can then change the postcondition of parse_commit_in_graph() to allow 'maybe_tree' to be NULL. If the tree is accessed later, we can load the tree's OID from the commit-graph in constant time and perform the lookup_tree().

On the Linux repository, performance tests were run for the following command:

    git log --graph --oneline -1000

    Before: 0.92s
    After:  0.66s
    Rel %: -28.3%

Adding '-- kernel/' to the command requires loading the root tree for every commit that is walked. There was no measureable performance change as a result of this patch.

This patch series depends on v7 of ds/commit-graph.

Derrick Stolee (4):
treewide: rename tree to maybe_tree
commit: create get_commit_tree() method
treewide: replace maybe_tree with accessor methods
commit-graph: lazy-load trees for commits

 blame.c                         | 18 +++++++++---------
 builtin/checkout.c              | 18 ++++++++++++------
 builtin/diff.c                  |  2 +-
 builtin/fast-export.c           |  6 +++---
 builtin/log.c                   |  4 ++--
 builtin/reflog.c                |  2 +-
 commit-graph.c                  | 27 +++++++++++++++++++++++----
 commit-graph.h                  |  7 +++++++
 commit.c                        | 18 +++++++++++++++++-
 commit.h                        |  5 ++++-
 contrib/coccinelle/commit.cocci | 30 ++++++++++++++++++++++++++++++
 fsck.c                          |  8 +++++---
 http-push.c                     |  2 +-
 line-log.c                      |  4 ++--
 list-objects.c                  | 10 +++++-----
 log-tree.c                      |  6 +++---
 merge-recursive.c               |  5 +++--
 notes-merge.c                   |  9 +++++----
 packfile.c                      |  2 +-
 pretty.c                        |  5 +++--
 ref-filter.c                    |  2 +-
 revision.c                      |  8 ++++----
 sequencer.c                     | 12 ++++++------
 sha1_name.c                     |  2 +-
 tree.c                          |  4 ++--
 walker.c                        |  2 +-
 26 files changed, 152 insertions(+), 66 deletions(-)
 create mode 100644 contrib/coccinelle/commit.cocci

2.17.0