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

Documentation/git.txt

@@ -544,6 +544,10 @@ other
 	a pager.  See also the `core.pager` option in
 	linkgit:git-config[1].
 
+`GIT_PROGRESS_DELAY`::
+	A number controlling how many seconds to delay before showing
+	optional progress indicators. Defaults to 2.
+
 `GIT_EDITOR`::
 	This environment variable overrides `$EDITOR` and `$VISUAL`.
 	It is used by several Git commands when, on interactive mode,