gitk: use git-diff-tree --no-commit-id

gitk: use git-diff-tree --no-commit-id

gitk switched to use git-diff-tree with one argument in gettreediffs and
getblobdiffs.  git-diff-tree with one argument outputs commit ID in from
of the patch.  This causes an empty line after "Comments" in the lower
right pane.  Also, the diff in the lower left pane has the commit ID,
which is useless there.

This patch makes git use the newly added -no-commit-id option for
git-diff-tree to suppress commit ID.  It also removes the p variable in
both functions, since it has become useless after switching to the
one-argument invocation for git-diff-tree.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>