Teach JGit to write a commit-graph formatted file by walking commit graph from specified commit objects. See: https://git-scm.com/docs/commit-graph-format/2.19.0 Bug: 574368 Change-Id: I34f9f28f8729080c275f86215ebf30b2d05af41d Signed-off-by: kylezhao <kylezhao@tencent.com>