For stable commits, use the same timestamp as the CLI tests do.
This preserves that commits that are created in the Signed References
module appears as "now" in the output.
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