gitk: Fix diff this->selected and selected->this functions

gitk: Fix diff this->selected and selected->this functions

The change in 8b7e5d76e836396a097bb6f61cf930ea872a7bd3, which makes
a couple of git-diff-tree calls supply only one id rather than two,
fixes the display when showing what a single commit did with dense
revlists, but broke the diff this->selected and diff selected->this
right-click menu functions.

Yann Dirson pointed this out and had a patch that fixed the diff
menu functions by passing a "singlecommit" flag around.  This fixes
it a bit differently, by making the ids and diffids variables be
either a single id, in the case of showing what a commit did, or
{oldid newid}, in the case of the diff menu functions.  That way
we can just pass $ids to git-diff-tree as is.  Most of the changes
in fact are just reversing the order of ids in $ids and $diffids,
because they used to be {child parent}, but git-diff-tree requires
old id before new id.

Signed-off-by: Paul Mackerras <paulus@samba.org>