commit-graph: avoid leaking topo_levels slab in write_commit_graph()

write_commit_graph initialises topo_levels using init_topo_level_slab(),
next it calls compute_topological_levels() which can cause the slab to
grow, we therefore need to clear the slab again using
clear_topo_level_slab() when we're done.

First introduced in 72a2bfc - which
is currently only in master and not on maint.

LeakSanitizer output:

==1026==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 8 byte(s) in 1 object(s) allocated from:
    #0 0x498ae9 in realloc /src/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:164:3
    #1 0xafbed8 in xrealloc /src/git/wrapper.c:126:8
    #2 0x7966d1 in topo_level_slab_at_peek /src/git/commit-graph.c:71:1
    #3 0x7965e0 in topo_level_slab_at /src/git/commit-graph.c:71:1
    #4 0x78fbf5 in compute_topological_levels /src/git/commit-graph.c:1472:12
    #5 0x78c5c3 in write_commit_graph /src/git/commit-graph.c:2456:2
    #6 0x535c5f in graph_write /src/git/builtin/commit-graph.c:299:6
    #7 0x5350ca in cmd_commit_graph /src/git/builtin/commit-graph.c:337:11
    git#8 0x4cddb1 in run_builtin /src/git/git.c:453:11
    git#9 0x4cabe2 in handle_builtin /src/git/git.c:704:3
    git#10 0x4cd084 in run_argv /src/git/git.c:771:4
    git#11 0x4ca424 in cmd_main /src/git/git.c:902:19
    git#12 0x707fb6 in main /src/git/common-main.c:52:11
    git#13 0x7fee4249383f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2083f)

Indirect leak of 524256 byte(s) in 1 object(s) allocated from:
    #0 0x498942 in calloc /src/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:154:3
    #1 0xafc088 in xcalloc /src/git/wrapper.c:140:8
    #2 0x796870 in topo_level_slab_at_peek /src/git/commit-graph.c:71:1
    #3 0x7965e0 in topo_level_slab_at /src/git/commit-graph.c:71:1
    #4 0x78fbf5 in compute_topological_levels /src/git/commit-graph.c:1472:12
    #5 0x78c5c3 in write_commit_graph /src/git/commit-graph.c:2456:2
    #6 0x535c5f in graph_write /src/git/builtin/commit-graph.c:299:6
    #7 0x5350ca in cmd_commit_graph /src/git/builtin/commit-graph.c:337:11
    git#8 0x4cddb1 in run_builtin /src/git/git.c:453:11
    git#9 0x4cabe2 in handle_builtin /src/git/git.c:704:3
    git#10 0x4cd084 in run_argv /src/git/git.c:771:4
    git#11 0x4ca424 in cmd_main /src/git/git.c:902:19
    git#12 0x707fb6 in main /src/git/common-main.c:52:11
    git#13 0x7fee4249383f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2083f)

SUMMARY: AddressSanitizer: 524264 byte(s) leaked in 2 allocation(s).

commit-graph.c

@@ -2471,6 +2471,7 @@ int write_commit_graph(struct object_directory *odb,
 	free(ctx->graph_name);
 	free(ctx->commits.list);
 	oid_array_clear(&ctx->oids);
+	clear_topo_level_slab(&topo_levels);
 
 	if (ctx->commit_graph_filenames_after) {
 		for (i = 0; i < ctx->num_commit_graphs_after; i++) {