commit-graph: clear oidset after finishing write

In graph_write() we store commits in an oidset, but never clean it up,
leaking the contents. We should clear it in the cleanup section.

The oidset comes from 6830c36077 (commit-graph.h: replace 'commit_hex'
with 'commits', 2020-04-13), but it was just replacing a string_list
that was also leaked. Curiously, we fixed the leak of some adjacent
variables in commit fa8953cb40 (builtin/commit-graph.c: extract
'read_one_commit()', 2020-05-18), but the oidset wasn't included for
some reason.

In combination with the preceding commits, this lets us mark t5324 as
leak-free.

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

builtin/commit-graph.c

@@ -311,6 +311,7 @@ cleanup:
 	FREE_AND_NULL(options);
 	string_list_clear(&pack_indexes, 0);
 	strbuf_release(&buf);
+	oidset_clear(&commits);
 	return result;
 }
 

t/t5324-split-commit-graph.sh

@@ -1,6 +1,8 @@
 #!/bin/sh
 
 test_description='split commit graph'
+
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 GIT_TEST_COMMIT_GRAPH=0