commit-graph.c: ensure graph layers respect core.sharedRepository

commit-graph.c: ensure graph layers respect core.sharedRepository

Non-layered commit-graphs use 'adjust_shared_perm' to make the
commit-graph file readable (or not) to a combination of the user, group,
and others.

Call 'adjust_shared_perm' for split-graph layers to make sure that these
also respect 'core.sharedRepository'. The 'commit-graph-chain' file
already respects this configuration since it uses
'hold_lock_file_for_update' (which calls 'adjust_shared_perm' eventually
in 'create_tempfile_mode').

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>