gitk: Show diffs for boundary commits

gitk: Show diffs for boundary commits

With this we run git-diff-tree on a commit even if we think it has
no parents, either because it really has no parents or because it
is a boundary commit.  This means that gitk shows the diff for a
boundary commit when it is selected.

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