In order to allow automatic migration to the feature level 'parent',
introduce a way to force writing signed references, even if the
user-controlled refs are unchanged.
To detect the special case of a Signed References history with a single
and root commit (which would otherwise be detected as feature level
'parent', even though the `refs/rad/sigrefs-parent` ref cannot be
written), keep information about the parent commit in `SignedRefs`.
The output of `rad inspect --sigrefs` is changed to match the more
strict interpretation of the feature level for histories with a single
and root commit.
In order for users to be able to inspect the feature level of Signed
References in a repository, have `rad inspect --sigrefs` attempt to
load Signed References, and, if successful, print the feature level.
Lorenz Leutgeb