`log -s/--summary` and `log --git` without `-p` don't do anything. I
also don't think it's very useful to pass these flags in an alias,
where you would then sometimes also pass `-p` to see the diff summary
in the output. We already have the `diff.format` config for that use
case. So let's make both of these flags imply `-p`.
I implemented it by making the `diff_format` variable an
`Option<DiffFormat>`, which is set iff we should show a patch. That
way we have the condition in one place, and the places we use it
cannot forget to check it.