commit: integrate commit graph with commit parsing

Teach Git to inspect a commit graph file to supply the contents of a
struct commit when calling parse_commit_gently(). This implementation
satisfies all post-conditions on the struct commit, including loading
parents, the root tree, and the commit date.

If core.commitGraph is false, then do not check graph files.

In test script t5318-commit-graph.sh, add output-matching conditions on
read-only graph operations.

By loading commits from the graph instead of parsing commit buffers, we
save a lot of time on long commit walks. Here are some performance
results for a copy of the Linux repository where 'master' has 678,653
reachable commits and is behind 'origin/master' by 59,929 commits.

| Command                          | Before | After  | Rel % |
|----------------------------------|--------|--------|-------|
| log --oneline --topo-order -1000 |  8.31s |  0.94s | -88%  |
| branch -vv                       |  1.02s |  0.14s | -86%  |
| rev-list --all                   |  5.89s |  1.07s | -81%  |
| rev-list --all --objects         | 66.15s | 58.45s | -11%  |

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/commit-graph.h b/commit-graph.h
index 2528478..73b28be 100644
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -5,6 +5,18 @@
 
 char *get_commit_graph_filename(const char *obj_dir);
 
+/*
+ * Given a commit struct, try to fill the commit struct info, including:
+ *  1. tree object
+ *  2. date
+ *  3. parents.
+ *
+ * Returns 1 if and only if the commit was found in the packed graph.
+ *
+ * See parse_commit_buffer() for the fallback after this call.
+ */
+int parse_commit_in_graph(struct commit *item);
+
 struct commit_graph {
 	int graph_fd;