Use the new object_type enumeration names within the codebase.

When `git_repository_set_head` is provided a remote reference, update
the reflog with the tag name, like we do with a branch.  This helps
consumers match the semantics of `git checkout remote`.

When `git_repository_set_head` is provided a tag reference, update the
reflog with the tag name, like we do with a branch.  This helps
consumers match the semantics of `git checkout tag`.

This also brings the soft-reset tests back to life. The function name
was missing an underscore, meaning they had not been running.

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.

Given HEAD -> master -> foo, when updating foo's reflog we should also
update HEAD's, as it's considered the current branch.

The reflog append function was overzealous in its checking. When passed
an old and new ids, it should not do any checking, but just serialize
the data to a reflog entry.