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.

On bare by default, or when core.logallrefupdates is false, we must not
write the reflog.

If the caller wants to update a ref to point to the same target as it
currently has, we should return early and avoid writing to the reflog.

When we update the current branch, we must also append to HEAD's reflog
to keep them in sync.

This is a bit of a hack, but as git.git says, it covers 100% of
default cases.

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.

The frontend used to look at the file directly, but that's obviously not
the right thing to do. Expose it on the backend and use that function
instead.

Sometimes (e.g. stash) we want to make sure that a log will be written,
even if it's not in one of the standard locations. Let's make that
easier.

git-core only writes to the reflogs of HEAD, refs/heads/ and,
refs/notes/ or if there is already a reflog in place. Adjust our code to
follow these semantics.

This was a convenience method for the refs front-end to do the reflog
writing. This is now done in the backend and it has no more reason for
being.