do_one_ref(): save and restore value of current_ref

If do_one_ref() is called recursively, then the inner call should not
permanently overwrite the value stored in current_ref by the outer
call.  Aside from the tiny optimization loss, peel_ref() expects the
value of current_ref not to change across a call to peel_entry().  But
in the presence of replace references that assumption could be
violated by a recursive call to do_one_ref:

do_for_each_entry()
  do_one_ref()
    builtin/describe.c:get_name()
      peel_ref()
        peel_entry()
          peel_object ()
            deref_tag_noverify()
              parse_object()
                lookup_replace_object()
                  do_lookup_replace_object()
                    prepare_replace_object()
                      do_for_each_ref()
                        do_for_each_entry()
                          do_for_each_entry_in_dir()
                            do_one_ref()

The inner call to do_one_ref() was unconditionally setting current_ref
to NULL when it was done, causing peel_ref() to perform an invalid
memory access.

So change do_one_ref() to save the old value of current_ref before
overwriting it, and restore the old value afterward rather than
setting it to NULL.

Reported by: Mantas Mikulėnas <grawity@gmail.com>

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'mh/packed-refs-various' into pu

* mh/packed-refs-various:
  do_one_ref(): save and restore value of current_ref