This fixes "git log --follow" to hopefully not leak memory any more, and
also cleans it up a bit to look more like some of the other functions that
use "diff_queued_diff" (by *not* using it directly as a global in the
code, but by instead just taking a pointer to the diff queue and using
that).

As to "diff_queued_diff", I think it would be better off not as a global
at all, but as being just an entry in the "struct diff_options" structure,
but that's a separate issue, and there may be some subtle reason for why
it's currently a global.

Anyway, no real changes. Instead of having a magical first entry in the
diff-queue, we now end up just keeping the diff-queue clean, and keeping
our "preferred" file pairing in an internal "choice" variable. That makes
it easy to switch the choice around when we find a better one.
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>