[BUG] git-diff-tree or git-commit: wrong diff(stat)

git-commit.sh uses git-diff-tree [options] HEAD --.  The '--' was
added in 521f9c4d to prevent problems when HEAD exists as a file but
git-diff-tree shows only differences below the current directory when
'--' is used.  Hence git-commit in a subdirectory shows only stats for
the subdirectory.  Example:
~/foo% git init
Initialized empty Git repository in .git/
~/foo% mkdir d
~/foo% seq 1 10 | tee a > d/b
~/foo% git add a d/b
~/foo% cd d
~/foo/d% git commit -m blub
Created initial commit 49d6f2eac51d3a2665a539adc2cc8676e5b14a3d
 1 files changed, 10 insertions(+), 0 deletions(-)
 create mode 100644 d/b