Merge branch 'ds/close-object-store' into maint

The commit-graph file is now part of the "files that the runtime
may keep open file descriptors on, all of which would need to be
closed when done with the object store", and the file descriptor to
an existing commit-graph file now is closed before "gc" finalizes a
new instance to replace it.

* ds/close-object-store:
  packfile: rename close_all_packs to close_object_store
  packfile: close commit-graph in close_all_packs
  commit-graph: use raw_object_store when closing
  commit-graph: extract write_commit_graph_file()
  commit-graph: extract copy_oids_to_commits()
  commit-graph: extract count_distinct_commits()
  commit-graph: extract fill_oids_from_all_packs()
  commit-graph: extract fill_oids_from_commit_hex()
  commit-graph: extract fill_oids_from_packs()
  commit-graph: create write_commit_graph_context
  commit-graph: remove Future Work section
  commit-graph: collapse parameters into flags
  commit-graph: return with errors during write
  commit-graph: fix the_repository reference

Documentation/technical/commit-graph.txt

@@ -127,23 +127,6 @@ Design Details
   helpful for these clones, anyway. The commit-graph will not be read or
   written when shallow commits are present.
 
-Future Work
------------
-
-- After computing and storing generation numbers, we must make graph
-  walks aware of generation numbers to gain the performance benefits they
-  enable. This will mostly be accomplished by swapping a commit-date-ordered
-  priority queue with one ordered by generation number. The following
-  operations are important candidates:
-
-    - 'log --topo-order'
-    - 'tag --merged'
-
-- A server could provide a commit-graph file as part of the network protocol
-  to avoid extra calculations by clients. This feature is only of benefit if
-  the user is willing to trust the file, because verifying the file is correct
-  is as hard as computing it from scratch.
-
 Related Links
 -------------
 [0] https://bugs.chromium.org/p/git/issues/detail?id=8