Handle git merge commits

[matthijskooijman]

Currently this tool requires a linear history, git merges confuse the line analyzer and cause errors. By tracking the parents of a merge commit (and also adapting the diff format for merges which has multiple added/deleted/context columns), it should be possible to do better analysis even with merges.

While coming back to my code this week after not looking at it for years, I found a pile of commits and some uncomitted code that intends to handle merge commits. I am not sure if it is actually complete, but on a quick test it does seem to work (no errors, no clue if the output is correct).

I've pushed all the code out to the https://github.com/matthijskooijman/patchdeps/tree/wip-handle-merge-commits branch, in case anyone is interested in this feature (might be me if I ever need this tool again, but not now. Maybe @pmhahn?). This branch might need some careful rebasing, since it was made (way before) the refactoring in #2.