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.

Don't require the branch to rebase, if given `NULL`, simply look up
`HEAD`.

Rename git_merge_head to git_annotated_commit, as it becomes used
in more operations than just merge.

Introduce `git_rebase` to set up a rebase session that can
then be continued.  Immediately, only merge-type rebase is
supported.