Removing a reflog upon ref deletion is something which only some
backends might wish to do. Backends which are database-backed may wish
to archive a reflog, log-based ones may not need to do anything.
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.
Any well-behaved program should write a descriptive message to the
reflog whenever it updates a reference. Let's make this more prominent
by removing the version without the reflog parameters.
