Fix issue with DiffIndex on initial commit (#11677)
Unfortunately #11614 introduced a bug whereby the initial commit of a
repository could not be seen due to there being no parent commit to
create a clear diff from.
Here we create a diffstat from the difference between the parentless SHA and the SHA of the empty tree - a constant known to git. (With thanks to @L0veSunshine for informing me of this SHA)
Thanks to @a1012112796 for initial attempt to fix.
Signed-off-by: Andrew Thornton <email@example.com>
Co-Authored-By: L0veSunshine <firstname.lastname@example.org>