Fix scroll position when entering staging view by clicking in the main view (#3992)HEADmaster

This is both a bug fix and a behavior change:

- The bug fix is that when entering the staging view for a selected file
by clicking in the main view, and the main view was scrolled down, then
it would scroll to the top and show an ugly range selection from the
line that was clicked to the line that is now under the mouse after
scrolling up.
- The behavior change is that when leaving the staging view by pressing
escape, it now retains its scroll position rather than scrolling back up
to the top.

In addition, this fixes a minor flickering issue when leaving the
staging view.

Note that maintaining the scroll position when going into and out of the
staging view is not always perfect for two reasons:
- the focused staging view does not wrap long lines, but the unfocused
one does
- a pager like delta might change the number of lines e.g. of the diff
hunks

For the second problem there's little we can do, but the first one will
be improved once we wrap lines in the focused staging view (see #1384
and #3558), which I'm currently working on.

- **Please check if the PR fulfills these requirements**

* [x] Cheatsheets are up-to-date (run `go generate ./...`)
* [x] Code has been formatted (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#code-formatting))
* [ ] Tests have been added/updated (see
[here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)
for the integration test guide)
* [ ] Text is internationalised (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#internationalisation))
* [ ] If a new UserConfig entry was added, make sure it can be
hot-reloaded (see
[here](https://github.com/jesseduffield/lazygit/blob/master/docs/dev/Codebase_Guide.md#using-userconfig))
* [ ] Docs have been updated if necessary
* [x] You've read through your own file changes for silly mistakes etc

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