git-diff.txt: add section "output format" describing the diff formats
git-diff.txt includes diff-options.txt which for the -p option refers
to a section "generating patches.." which is missing from the git-diff
documentation. This patch adapts diff-format.txt to additionally
mention the git-diff program, and includes diff-format.txt into
git-diff.txt.
Tino Keitel noticed this problem.
Signed-off-by: Gerrit Pape <pape@smarden.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>