This change makes revwalks a bit faster by using the `commit-graph` file
(if present). This is thanks to the `commit-graph` allow much faster
parsing of the commit information by requiring near-zero I/O (aside from
reading a few dozen bytes off of a `mmap(2)`-ed file) for each commit,
instead of having to read the ODB, inflate the commit, and parse it.

This is done by modifying `git_commit_list_parse()` and letting it use
the ODB-owned commit-graph file.

Part of: #5757