commit-graph: free all elements of graph chain

commit-graph: free all elements of graph chain

When running "commit-graph verify", we call free_commit_graph(). That's
sufficient for the case of a single graph file, but if we loaded a chain
of split graph files, they form a linked list via the base_graph
pointers. We need to free all of them, or we leak all but the first
struct.

We can make this work by teaching free_commit_graph() to walk the
base_graph pointers and free each element. This in turn lets us simplify
close_commit_graph(), which does the same thing by recursion (we cannot
just use close_commit_graph() in "commit-graph verify", as the function
takes a pointer to an object store, and the verify command creates a
single one-off graph struct).

While indenting the code in free_commit_graph() for the loop, I noticed
that setting g->data to NULL is rather pointless, as we free the struct
a few lines later. So I cleaned that up while we're here.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>