commit-graph: free write-context entries before overwriting

commit-graph: free write-context entries before overwriting

When writing a split graph file, we replace the final element of the
commit_graph_hash_after and commit_graph_filenames_after arrays. But
since these are allocated strings, we need to free them before
overwriting to avoid leaking the old string.

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