lib/repo: Create repo directories as 0775

For repo structure directories like `objects`, `refs`, etc... we should
be more permissive and let the system's `umask` narrow down the
permission bits as wanted.

This came up in a context where we want to be able to have read/write
access on an OSTree repo on NFS from two separate OpenShift apps by
using supplemental groups[1] so we don't require SCCs for running as the
same UID (supplemental groups are part of the default restricted SCC).

[1] https://docs.openshift.com/container-platform/3.11/install_config/persistent_storage/persistent_storage_nfs.html#nfs-supplemental-groups

src/libostree/ostree-core-private.h

@@ -30,6 +30,12 @@ G_BEGIN_DECLS
 /* It's what gzip does, 9 is too slow */
 #define OSTREE_ARCHIVE_DEFAULT_COMPRESSION_LEVEL (6)
 
+/* Note the permissive group bits. We want to be liberal here and let individual machines
+ * narrow permissions as needed via umask. This is important in setups where group ownership
+ * can matter for repo management (like OpenShift). */
+#define DEFAULT_DIRECTORY_MODE 0775
+#define DEFAULT_REGFILE_MODE 0660
+
 /* This file contains private implementation data format definitions
  * read by multiple implementation .c files.
  */