Merge branch 'tb/fix-persistent-shallow' into master

When "fetch.writeCommitGraph" configuration is set in a shallow
repository and a fetch moves the shallow boundary, we wrote out
broken commit-graph files that do not match the reality, which has
been corrected.

* tb/fix-persistent-shallow:
  commit.c: don't persist substituted parents when unshallowing

object.h

@@ -25,6 +25,7 @@ struct parsed_object_pool {
 	char *alternate_shallow_file;
 
 	int commit_graft_prepared;
+	int substituted_parent;
 
 	struct buffer_slab *buffer_slab;
 };