will be constructed, which has nr_parent < 0 to make it easier
to discern from user provided grafts.
+Note that the shallow feature could not be changed easily to
+use replace refs: a commit containing a `mergetag` is not allowed
+to be replaced, not even by a root commit. Such a commit can be
+made shallow, though. Also, having a `shallow` file explicitly
+listing all the commits made shallow makes it a *lot* easier to
+do shallow-specific things such as to deepen the history.
+
Since fsck-objects relies on the library to read the objects,
it honours shallow commits automatically.