commit-graph.c: make 'commit-graph-chain's read-only

commit-graph.c: make 'commit-graph-chain's read-only

In a previous commit, we made incremental graph layers read-only by
using 'git_mkstemp_mode' with permissions '0444'.

There is no reason that 'commit-graph-chain's should be modifiable by
the user, since they are generated at a temporary location and then
atomically renamed into place.

To ensure that these files are read-only, too, use
'hold_lock_file_for_update_mode' with the same read-only permission
bits, and let the umask and 'adjust_shared_perm' take care of the rest.

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