plumbing: commitgraph, Add generation v2 support

This PR adds in support for generation v2 support and a couple of new
walkers to match --date-order etc options on log. This PR also fixes a
bug in the chain code and adds more tests.

Signed-off-by: Andrew Thornton <art27@cantab.net>

1 files changed, 7 insertions, 0 deletions

diff --git a/plumbing/object/commitgraph/commitnode_graph.go b/plumbing/object/commitgraph/commitnode_graph.go
index 252b518..0f51e3b 100644
--- a/plumbing/object/commitgraph/commitnode_graph.go
+++ b/plumbing/object/commitgraph/commitnode_graph.go
@@ -120,6 +120,13 @@ func (c *graphCommitNode) Generation() uint64 {
 	return c.commitData.Generation
 }
 
+func (c *graphCommitNode) GenerationV2() uint64 {
+	// If the commit-graph file was generated with older Git version that
+	// set the generation to zero for every commit the generation assumption
+	// is still valid. It is just less useful.
+	return c.commitData.GenerationV2
+}
+
 func (c *graphCommitNode) Commit() (*object.Commit, error) {
 	return object.GetCommit(c.gci.s, c.hash)
 }