Merge branch 'ds/commit-graph-on-fetch' into jch

"git commit-grph write" hit BUG() in corner cases.

* ds/commit-graph-on-fetch:
  commit-graph: fix writing first commit-graph during fetch
  t5510-fetch.sh: demonstrate fetch.writeCommitGraph bug

diff --cc commit-graph.c
Simple merge

diff --cc t/t5510-fetch.sh
Simple merge