ui-patch: match git-format-patch(1) output

Using (DIFF_FORMAT_DIFFSTAT | DIFF_FORMAT_PATCH) causes Git to emit a
"---" line between the commit message and the body of the patch, which
fixes a regression introduced in commit 455b598 (ui-patch.c: Use
log_tree_commit() to generate diffs, 2013-08-20), prior to which we
inserted the "---" line ourselves.

DIFF_FORMAT_SUMMARY is added so that we match the output of
git-format-patch(1) without the "-p" option.

Signed-off-by: John Keeping <john@keeping.me.uk>