Merge branch 'dl/reflog-with-single-entry'

After expiring a reflog and making a single commit, the reflog for
the branch would record a single entry that knows both @{0} and
@{1}, but we failed to answer "what commit were we on?", i.e. @{1}

* dl/reflog-with-single-entry:
  refs: allow @{n} to work with n-sized reflog
  refs: factor out set_read_ref_cutoffs()