`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.
"log -p | less" is the option I often use with hg/git to find interesting
bits from the changelog, and I think it's also valid with jj. Unlike
"hg log -p --stat", "jj log -p --summary" does not show both diff summary
and patch to reflect the internal structure. This behavoir is arguable and
may be changed later.
The logic of show_patch() is extracted from cmd_show().
