Merge branch 'es/format-patch-interdiff'
"git format-patch" learned a new "--interdiff" option to explain
the difference between this version and the previous atttempt in
the cover letter (or after the tree-dashes as a comment).
* es/format-patch-interdiff:
format-patch: allow --interdiff to apply to a lone-patch
log-tree: show_log: make commentary block delimiting reusable
interdiff: teach show_interdiff() to indent interdiff
format-patch: teach --interdiff to respect -v/--reroll-count
format-patch: add --interdiff option to embed diff in cover letter
format-patch: allow additional generated content in make_cover_letter()