A shallow repository can currently only be completely unshallowed, which
is caused by mark_local() only marking locally-existing objects as
wanted if the fetch depth is set to INT_MAX (GIT_FETCH_DEPTH_UNSHALLOW).
This prevents deepening the history of a shallow clone to an arbitrary
number of commits, which may be preferable over full unshallowing for
large repositories.

Enable deepening and shortening shallow clones by marking
locally-existing objects as wanted whenever the fetch depth is set to
any non-default value (either GIT_FETCH_DEPTH_UNSHALLOW or an arbitrary
positive integer).