Fix diff-cover with recent git versions on shallow repositories (#26879)
Fix diff-cover with recent git versions on shallow repositories (#26879)

When we tried upgrading Jenkins workers from Ubuntu 16.04 to 20.04, `diff-cover` started silently failing with a return code of `1`.  We suspect this is due to https://github.com/Bachmann1234/diff_cover/issues/153 , so we're adding the workaround described in that ticket.
arbrandes/edx-platform,angelapper/edx-platform,eduNEXT/edx-platform,EDUlib/edx-platform,angelapper/edx-platform,arbrandes/edx-platform,edx/edx-platform,arbrandes/edx-platform,edx/edx-platform,arbrandes/edx-platform,eduNEXT/edunext-platform,eduNEXT/edunext-platform,eduNEXT/edunext-platform,eduNEXT/edx-platform,EDUlib/edx-platform,EDUlib/edx-platform,angelapper/edx-platform,eduNEXT/edunext-platform,eduNEXT/edx-platform,eduNEXT/edx-platform,EDUlib/edx-platform,edx/edx-platform,edx/edx-platform,angelapper/edx-platform