Use VS Code 3 Way Merge Editor as git merge tool

[hediet]

This should be supported:

git config --global merge.tool vscode
git config --global mergetool.vscode.cmd 'code --merge $BASE $LOCAL $REMOTE $MERGED'

So that in a case of a merge conflict

git mergetool

Can be used.

[jrieken]

Yeah, that should be similar to the --wait command line flag that we already have

[bpasero]

💯

I think this is a duplicate of #5770

This probably needs a combination of --wait on the result file and support to pass in an argument to enforce opening the merge editor. We do something similar with --diff so I would try to go from there: with --diff you pass in 2 paths and we end up opening a diff editor instead of opening the 2 paths as editors separately.

vscode/src/vs/platform/window/common/window.ts

Line 241 in e775136

readonly filesToDiff?: IPathData[];

We need to handle this request both for when a new window opens as initial editors to open but also when opening into an existing window, which is the more typical case for when using git while VSCode is already running:

vscode/src/vs/workbench/browser/layout.ts

Line 456 in e775136

const initialFilesToOpen = this.getInitialFilesToOpen();

vscode/src/vs/workbench/electron-sandbox/window.ts

Line 185 in e775136

ipcRenderer.on('vscode:openFiles', (event: unknown, request: IOpenFileRequest) => this.onOpenFiles(request));

A little challenge is that merge editor is a contrib while handling of arguments currently does not have a contribution model so we might want to think about moving the merge editor to core or would have to add a hack to call a contributed command.

[joaomoreno]

Let's use #5770 for this, so all those people get pinged.