Searching in the "Divergence from upstream" view would select the wrong lines.
The OnSearchSelect function gets passed a view index, and uses it to select a
model line. In most views these are the same, but not in the divergence view
(because of the Remote/Local section headers).

We do show the graph in the left/right view, as of b767357.

We want to add an additional method to ISearchableContext later in this branch,
and this will make sure that we don't forget to implement it in any concrete
context.

Just to be really sure that it not only contains the expected status text, but
also actually shows it.

…ighting

Previously, jumping to the next search result with 'n' would not show the search
result in the selected line. This is especially annoying when there is more than
one search result on a single line, because pressing 'n' seemingly does nothing.

Now we highlight the search result on top of the selection highlight, which
solves the problem nicely.

We will need it again when searching the model; extracting the change as a
separate commit just to make the diffs of the following commits smaller.

When we add support for searching the model, the length of a highlighted search
result isn't necessarily the same as the length of the search string. As an
example, consider that you are searching for a full commit hash; the view only
renders an abbreviated hash, so when the hash is found in the model, we only
want to highlight the abbreviated version.

Prepare for this by adding an XEnd field to cellPos; when searching the view,
this is always XStart + len(searchString).

Also, rename cellPos to SearchPosition and uppercase its fields to expose them;
we will use the struct in the API to set a model search function in the next
commit.

Add a method View.SetModelSearchFunc; if set, it will be used to look for search
results instead of searching the view lines.

This will be needed when we optimize some list views to render only the visible
portion of the view instead of all lines of the list. However, it also has the
benefit that it allows searching for the full text of something that is only
rendered in an abbreviated form in the view; the most notable example being
commit hashes.

This is opt-in, so we still support searching the view as before. This is needed
for two reasons:
- adding a model search func can be a bit of work, so we should only do it when
  there's a benefit
- some views don't even have a model

…ruction

There is no need to overwrite things with spaces; since tcell clears the whole
surface before drawing, it is enough to just truncate the line at the current
write position.

It is similar to OverwriteLines, except that it also clears all lines outside of
the area that was rerendered. This is needed for views that only draw the
visible portion of their model, to ensure that cells for invisble lines don't
accumulate as you scroll.

When refreshViewportOnChange is true, we would refresh the viewport once at the
end of FocusLine, and then we would check at the end of AfterLayout if the
origin has changed, and refresh again if so. That's unnecessarily complicated,
let's just unconditionally refresh at the end of AfterLayout only.

…dently

This is a commit only for testing; we'll drop it again before merging.

We now have two separate flags, refreshViewportOnChange and
renderOnlyVisibleLines, but we only have views that either use them both, or
neither. To test whether the logic around them is really independent, change two
other views to use them independently:

- the local branches view is changed to refresh viewport on change, and in order
  to make that testable, we change the branch render function so that the
  selected branch is rendered yellow. This is comparable to the graph node of
  the selected commit being rendered white.
- the remote branches view is changed to render only the visible lines.
  Conveniently, this is a view that uses filtering and not searching, so we
  don't have to bother implementing a model search func.