The only time we should call SetSelectedLineIdx is when we are happy for a
select range to be retained which means things like moving the selected line
index to top top/bottom or up/down a page as the user navigates.
But in every other case we should now call SetSelection because that will
set the selected index and cancel the range which is almost always what we
want.
The algorithm works by blaming the deleted lines, so if a hunk contains only
added lines, we can only hope that it also belongs in the same commit. Warn the
user about this.
Note: the warning might be overly agressive, we'll have to see if this is
annoying. The reason is that it depends on the diff context size whether added
lines go into their own hunk or are grouped together with other added or deleted
lines into one hunk. However, our algorithm uses a diff context size of 0,
because that makes it easiest to parse the diff; this results in hunks having
only added lines more often than what the user sees. For example, moving a line
of code down by two lines will likely result in a single hunk for the user, but
in two hunks for our algorithm. On the other hand, being this strict makes the
warning consistent. We could consider using the user's diff context size in the
algorithm, but then it would depend on the current context size whether the
warning appears, which could be confusing. Plus, it would make the algorithm
quite a bit more complicated.
