Merge branch 'js/commit-graph-warning'

When certain features (e.g. grafts) used in the repository are
incompatible with the use of the commit-graph, we used to silently
turned commit-graph off; we now tell the user what we are doing.

* js/commit-graph-warning:
  commit-graph: when incompatible with graphs, indicate why

diff --cc commit-graph.c
Simple merge