Instead of rebasing from the commit below the current one and then setting the
current one to "edit", we rebase from the current one and insert a "break" after
it. In most cases the behavior is exactly the same as before, except that the
new method also works if the current commit is a merge commit. This is useful if
you want to create a new commit at the very beginning of your branch (by editing
the last commit before your branch).
There are two reasons for doing this:
1. The view cursor position is often out of sync with the selected line; see
first commit of this branch.
2. The highlighting is already turned off when the view loses focus, and never
turned back on thereafter. So just turn it off from the start then.
We already have this very convenient behavior of jumping to the next stageable
line after staging something. However, while this worked well for staging
single lines or hunks, it didn't work correctly when staging a range of lines;
in this case we want to start searching from the first line of the range.
While we try to keep the view's cursor position in sync with the context state's
selectedLineIdx (at least when pressing up or down), there are enough situations
where the two run out of sync; for example when initially opening the view, or
after staging a hunk, or when scrolling the view using the wheel. While it would
be possible to fix these situations to keep them always in sync, it doesn't seem
worth it, because the view's cursor position isn't really used for anything
else. So we rather special-case the SelectedLine/SelectedLineIdx functions of
ViewDriver to query the context state's selectedLineIdx directly if it is a
patch explorer context.
There's no need for refreshing anything; all that's needed is to re-focus the
selected list item. This way it will also work in other panels, which we are
about to add in the next commit.
