Merge branch 'jc/format-patch-with-range-diff'

The inter/range-diff output has been moved to the end of the patch
when format-patch adds it to a single patch, instead of writing it
before the patch text, to be consistent with what is done for a
cover letter for a multi-patch series.

* jc/format-patch-with-range-diff:
  format-patch: move range/inter diff at the end of a single patch output
  show_log: factor out interdiff/range-diff generation