Codenotify seems to include diff of all parents when there are merge commits

[felixfbecker]

When a branch contains a merge commit, e.g. because main was merged into the branch to update it, Codenotify seems to not only analyze the diff of commits on the branch line, but also the diff of commits on main. Example: https://github.com/sourcegraph/sourcegraph/pull/17521#issuecomment-765274334

I haven't looked at the code, but Git provides various command parameters that control to only include one line of parents in git diff and git log that may help here.