Support Docker-style whiteouts

This is to enable importing Docker layers as ostree commits, then
checking them out in a union.

The prototype work for this is in:
https://github.com/cgwalters/dlayer-ostree

Though it will likely ultimately end up in:
https://github.com/projectatomic/atomic

src/libostree/ostree-repo.h

@@ -557,7 +557,8 @@ typedef struct {
   
   guint enable_uncompressed_cache : 1;
   guint disable_fsync : 1;
-  guint reserved : 30;
+  guint process_whiteouts : 1;
+  guint reserved : 29;
 
   const char *subpath;