format-patch: use 'origin' as start of current-series-range when known
When formatting a patch series over `origin..HEAD`, one would expect that range to be used as the current-series-range when computing a range-diff between the previous and current versions of a patch series. However, infer_range_diff_ranges() ignores `origin..HEAD` when --range-diff=<prev> specifies a single revision rather than a range, and instead unexpectedly computes the current-series-range based upon <prev>. Address this anomaly by unconditionally using `origin..HEAD` as the current-series-range regardless of <prev> as long as `origin` is known, and only fall back to basing current-series-range on <prev> when `origin` is not known. Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>