Fix Stashing partial files for git version >= 2.35.0 (#3569)

- **PR Description**
Use git's `--staged` flag to stash staged changes if available (requires
git 2.35.0 or later), and fall back to our previous method if not. This
is a lot faster than our previous method, and it fixes two bugs, see
linked issues.

Fixes #3333 and #3563.

- **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))
* [x] Tests have been added/updated (see
[here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)
for the integration test guide)
* [x] Text is internationalised (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#internationalisation))
* [x] Docs (specifically `docs/Config.md`) have been updated if
necessary
* [x] You've read through your own file changes for silly mistakes etc

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