format-patch: allow additional generated content in make_cover_letter()

format-patch: allow additional generated content in make_cover_letter()

make_cover_letter() returns early when it lacks sufficient state to emit
a diffstat, which makes it difficult to extend the function to reliably
emit additional generated content. Work around this shortcoming by
factoring diffstat-printing logic out to its own function and calling it
as needed without otherwise inhibiting normal control flow.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>