Merge branch 'jk/commit-graph-lazy-load-fallback' into jch

The logic to lazy-load trees from the commit-graph has been made
more robust by falling back to reading the commit object when
the commit-graph is no longer available.

* jk/commit-graph-lazy-load-fallback:
  commit: fall back to full read when maybe_tree is NULL

diff --cc commit.c
Simple merge