Properly handle multi-parent commits

[albx79]

In case of merge commits, what-bump always follows the first parent, which happens to be the main branch. This breaks when the release tag is on a separate branch: in this case what-bump will follow all commits on the main branch `til the root is found, then stops.

Ideally, what-bump should do what git does, and properly find a path from HEAD up to the revision specified on the command-line. However, I have no idea how to achieve this.

If multiple paths exist from HEAD to the target revision, then some way to disambiguate would be desirable (even though either erroring out or choosing an arbitrary path by default would be OK).