git-diff.txt: add section "output format" describing the diff formats

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>