commit-graph: when closing the graph, also release the slab

commit-graph: when closing the graph, also release the slab

The slab has information about the commit graph. That means that it is
meaningless (and even misleading) when the commit graph was closed.

This seems not to matter currently, but we're about to fix a
Windows-specific bug where `git pull` does not close the object store
before fetching (risking that an implicit auto-gc fails to remove the
now-obsolete pack file(s)), and once we have that bug fix in place, it
does matter: after that bug fix, we will open the object store, do some
stuff with it, then close it, fetch, and then open it again, and do more
stuff. If we close the commit graph without releasing the corresponding
slab, we're hit by a symptom like this in t5520.19:

	BUG: commit-reach.c:85: bad generation skip 9223372036854775807
	> 3 at 5cd378271655d43a3b4477520014f02213ad1546

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