By default all merges are shown but if "samebranch" is set to False then merges
with the same branch (i.e. where both parents belong to the same branch) will
be filtered out.
Conversely, if "samebranch" is set to True then only merges with the same branch
will be shown.
This is useful to visualize at a high level the relationships between different
branches and how they are merged with each other.
With the addition of the merge(withbranch) idiom on a previous revision this
could already be done in a quite complicated way, by doing something like:
merge() and branch(somebranch) and not merge(somebranch)
This is not very practical ano only works for a single branch. Thus this new
option is added.