Log message printout cleanups (#3): fix --pretty=oneline

Log message printout cleanups (#3): fix --pretty=oneline

This option is very special, since pretty_print_commit() will _remove_
the newline at the end of it, so we want to have an extra separator
between the things.

I added a honking big comment this time, so that (a) I don't forget this
_again_ (I broke "oneline" several times during this printout cleanup),
and so that people can understand _why_ the code does what it does.

Now, arguably the alternate fix is to always have the '\n' at the end in
pretty-print-commit, but git-rev-list depends on the current behaviour
(but we could have git-rev-list remove it, whatever).

With the big comment, the code hopefully doesn't get broken again. And now
things like

	git log --pretty=oneline --cc --patch-with-stat

works (even if that is admittedly a totally insane combination: if you
want the patch, having the "oneline" log format is just crazy, but hey,
it _works_. Even insane people are people).

Signed-off-by: Junio C Hamano <junkio@cox.net>