We have a bunch of APIs to do GPG verification of a commit,
but that doesn't generalize to signapi. Further, they
require the caller to check the signature status explicitly
which seems like a trap.
This much higher level API works with both GPG and signapi.
The intention is to use this in things that are doing "external
pulls" like the ostree-ext tar import support. There we will
get the commitmeta from the tarball and we want to verify it
at the same time we import the commit.
Colin Walters