Use git.NullSHA instead of hardcoded "00000000..."
Change various code to use git.NullSHA instead of the hardcoded "0000000..." value. I'm not e.g. raw diff-tree output in tests, just the Go code itself.
Change various code to use git.NullSHA instead of the hardcoded "0000000..." value. I'm not e.g. raw diff-tree output in tests, just the Go code itself.