This makes it so that when the staging view is resized, we keep the same patch
line selected (as opposed to the same view line, which may correspond to a
different patch line after resizing). It doesn't seem like a terribly important
feature for resizing the window, but it is essential when initially entering the
staging view: we select the first line of the first hunk in this case, but we do
that before layout runs. At layout time the view is then split into
unstaged/staged changes, and if this split is horizontal, the view gets narrower
and may be wrapped in a different way. With this commit we ensure that the first
line of the first hunk is still selected after that.
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