Fix gitk this->selected diffs

Fix gitk this->selected diffs

The change made in 8b7e5d76e836396a097bb6f61cf930ea872a7bd3 to
accomodate dense revlists in single-commit diffs has broken computing
of diffs between arbitrary trees, which does need to consider two
commit ids.

This patch changes the two git-diff-tree calls to get the necessary
two ids in this case.  It does so by propagating a "singlecommit" flag
through all functions involved via an additional argument.

Signed-off-by: Yann Dirson <ydirson@altern.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>