We want to mark all local branch heads with a "*" in the local commits panel, to
make it easier to see how branches are stacked onto each other. In order to not
confuse users with "*" markers that they don't understand, do this only for the
case where users actually use stacked branches; those users are likely not going
to be confused by the display. This means we want to filter out a few branch
heads that shouldn't get the marker: the current branch, any main branch, and
any old branch that has been merged to master already.
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