Support diff merges option in range diff

[dscho]

The git range-diff command does the same with merge commits as git rebase: It ignores them.

However, when comparing branch thickets it can be quite illuminating to watch out for inadvertent changes in merge commits, in particular when some "evil" merges have been replayed, i.e. merges that needed to introduce changes outside of the merge conflicts (e.g. when one branch changed a function's signature and another branch introduced a caller of said function), in case the replayed merge is no longer "evil" and therefore potentially incorrect.

Let's introduce support for the --diff-merges option that is passed through to those git log commands.