Now "git log" is in reusable shape, add "git whatchanged" which
essentially is a synonym with different default for people whose
fingers are already trained.
There is a subtle difference from the shell-script version; the
first line of each entry is now "commit <object name>", instead
of "diff-tree <object name> (from <object name>)." I suspect
that showing the parent name that way is useful, so this may be
something we would want to fix (the user can say --pretty=raw to
get that information but that is a bit ugly).