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.

Fix #11650

Closes #11674

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-Authored-By: L0veSunshine <xuan199651@gmail.com>