run_auto_maintenance(): implicitly close the object store

Before spawning the auto maintenance, we need to make sure that we
release all open file handles to all the `.pack` files (and MIDX files
and commit-graph files and...) so that the maintenance process has the
freedom to delete those files.

So far, we did this manually every time before calling
`run_auto_maintenance()`. With the new `close_object_store` flag, we can
do that implicitly in that function, which is more robust because future
callers won't be able to forget to close the object store.

Note: this changes behavior slightly, as we previously _always_ closed
the object store, but now we only close the object store when actually
running the auto maintenance. In practice, this should not matter (if
anything, it might speed up operations where auto maintenance is
disabled).

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -1848,7 +1848,6 @@ next:
 	 */
 	if (!state->rebasing) {
 		am_destroy(state);
-		close_object_store(the_repository->objects);
 		run_auto_maintenance(state->quiet);
 	}
 }

builtin/fetch.c

@@ -2133,8 +2133,6 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 					     NULL);
 	}
 
-	close_object_store(the_repository->objects);
-
 	if (enable_auto_gc)
 		run_auto_maintenance(verbosity < 0);
 

builtin/merge.c

@@ -469,7 +469,6 @@ static void finish(struct commit *head_commit,
 			 * We ignore errors in 'gc --auto', since the
 			 * user should see them.
 			 */
-			close_object_store(the_repository->objects);
 			run_auto_maintenance(verbosity < 0);
 		}
 	}

builtin/rebase.c

@@ -740,7 +740,6 @@ static int finish_rebase(struct rebase_options *opts)
 	delete_ref(NULL, "REBASE_HEAD", NULL, REF_NO_DEREF);
 	unlink(git_path_auto_merge(the_repository));
 	apply_autostash(state_dir_path("autostash", opts));
-	close_object_store(the_repository->objects);
 	/*
 	 * We ignore errors in 'git maintenance run --auto', since the
 	 * user should see them.

run-command.c

@@ -1891,6 +1891,7 @@ int run_auto_maintenance(int quiet)
 		return 0;
 
 	maint.git_cmd = 1;
+	maint.close_object_store = 1;
 	strvec_pushl(&maint.args, "maintenance", "run", "--auto", NULL);
 	strvec_push(&maint.args, quiet ? "--quiet" : "--no-quiet");