Merge branch 'es/format-patch-interdiff-cleanup'

"format-patch --range-diff=<prev> <origin>..HEAD" has been taught not to ignore <origin> when <prev> is a single version. * es/format-patch-interdiff-cleanup: format-patch: use 'origin' as start of current-series-range when known diff-lib: tighten show_interdiff()'s interface diff: move show_interdiff() from its own file to diff-lib