This allows to do the equivalent of "git rebase --onto <target> <base>", by
first marking the <base> commit with the new command, and then selecting the
target branch and invoking the usual rebase command there.
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.
Go really doesn't like us doing anything inheritance-y: it does not support open recursion meaning
it's really hard to re-use code. As such, here we're falling back to conditional logic.
This fixes an issue where our ListContextTrait was calling FocusLine which was intended to be
overridden by ViewportListContextTrait, but the subclassed function wasn't being called. I'm
not actually sure how this went wrong given that it was working fine in the past, but at any rate,
the new code is easy to follow.
This begins a big refactor of moving more code out of the Gui struct into contexts, controllers, and helpers. We also move some code into structs in the
gui package purely for the sake of better encapsulation
