Merge editor: preserve view state

[bpasero]

Testing #150389

Similar to #150791 where BaseTextEditor provides some helper methods for storing editor view state, the merge editor should associate view state like the diff editor does: e.g. selection, folding regions etc. so that they can restore after window reload.

One thing that is special about the diff editor: when you open a diff editor for the first time, we automatically reveal the first diff and ignore any previous view state. This helps focussing on what has actually changed. However, when a diff editor was opened before and is just made active again, we do restore the view state.

[jrieken]

To verify:

• open merge editor
• select something in input1, show the find widget in input2, etc
• reload the window or close/reopen the editor
• ensure that the view state is restored

---

To get a merge conflict and editor

• use this setting "git.mergeEditor": true
• run the commands below
• open the conflict from the SCM view

git clone https://github.com/hediet/vscode-3wm-test-repo.git
cd vscode-3wm-test-repo
git fetch --all
git checkout input1
git merge input2

[isidorn]

I can still reproduce this, reopening. I am following steps from the above comment. Here's a gif:

[jrieken]

Sorry forgot to mention that it needs todays insiders.

[jrieken]

Still doesn't work... Need to investigate., sorry for the fuzz

[jrieken]

Sorry, my verification steps are bad. The find widget isn't part of any editor view state, selecting and folding etc is. So

To verify:

• open merge editor
• select something in input1, select something in input2, fold something input3
• reload the window or close/reopen the editor
• ensure that the view state is restored