Merge branch 'ds/commit-graph-delay-gen-progress'

One kind of progress messages were always given during commit-graph
generation, instead of following the "if it takes more than two
seconds, show progress" pattern, which has been corrected.

* ds/commit-graph-delay-gen-progress:
  commit-graph: use start_delayed_progress()
  progress: create GIT_PROGRESS_DELAY

diff --cc Documentation/git.txt
Simple merge

diff --cc commit-graph.c
Simple merge

diff --cc t/t5318-commit-graph.sh
Simple merge