Fix embarrassing "git log --follow" bug

Fix embarrassing "git log --follow" bug

It turns out that I completely broke "git log --follow" with my recent
patch to revision.c ("Fix revision log diff setup, avoid unnecessary diff
generation", commit b7bb760d5ed4881422673d32f869d140221d3564).

Why? Because --follow obviously requires the diff machinery to function,
exactly the same way pickaxe does.

So everybody is away right now, but considering that nobody even noticed
this bug, I don't think it matters. But for the record, here's the trivial
one-liner fix (well, two, since I also fixed the comment).

Because of the nature of the bug, if you ask for patches when following
(which is one of the things I normally do), the bug is hidden, because
then the request for diff output will automatically also enable the diffs
themselves.

So while "git log --follow <filename>" didn't work, adding a "-p"
magically made it work again even without this fix.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>