refs: new public ref function: safe_create_reflog

The safe_create_reflog function creates a reflog, if it does not
already exist.

The log_ref_setup function becomes private and gains a force_create
parameter to force the creation of a reflog even if log_all_ref_updates
is false or the refname is not one of the special refnames.

The new parameter also reduces the need to store, modify, and restore
the log_all_ref_updates global before reflog creation.

In a moment, we will use this to add reflog creation commands to
git-reflog.

Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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