"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().