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>