Merge branch 'tb/commit-graph-perm-bits'

Merge branch 'tb/commit-graph-perm-bits'

Some of the files commit-graph subsystem keeps on disk did not
correctly honor the core.sharedRepository settings and some were
left read-write.

* tb/commit-graph-perm-bits:
  commit-graph.c: make 'commit-graph-chain's read-only
  commit-graph.c: ensure graph layers respect core.sharedRepository
  commit-graph.c: write non-split graphs as read-only
  lockfile.c: introduce 'hold_lock_file_for_update_mode'
  tempfile.c: introduce 'create_tempfile_mode'