Test some additional exotic rebase setup behavior: that we are
able to set up properly when already in a detached HEAD state,
that the caller specifies all of branch, upstream and onto,
and that the caller specifies branch, upstream and onto by ID.
We want to use the "checkout: moving from ..." message in order to let
git know when a change of branch has happened. Make the convenience
functions for this goal write this message.
The signature for the reflog is not something which changes
dynamically. Almost all uses will be NULL, since we want for the
repository's default identity to be used, making it noise.
In order to allow for changing the identity, we instead provide
git_repository_set_ident() and git_repository_ident() which allow a user
to override the choice of signature.
