Allow to return an exit code from `--wait` invocations

I'm using Sourcetree as a git GUI and tried to set-up vscode as a three-way merge tool. Sourcetree expects that the tool will return a 0 on merge success and some error code on failure/abort. However, vscode returns 0 almost immediately, causing Sourcetree to delete the merge files. <!-- ⚠️⚠️ Do Not Delete This! bug_report_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- 🕮 Read our guide about submitting issues: https://github.com/microsoft/vscode/wiki/Submitting-Bugs-and-Suggestions --> <!-- 🔎 Search existing issues to avoid creating duplicates. --> <!-- 🧪 Test using the latest Insiders build to see if your issue has already been fixed: https://code.visualstudio.com/insiders/ --> <!-- 💡 Instead of creating your report here, use 'Report Issue' from the 'Help' menu in VS Code to pre-fill useful information. --> <!-- 🔧 Launch with `code --disable-extensions` to check. --> Does this issue occur when all extensions are disabled?: Yes <!-- 🪓 If you answered No above, use 'Help: Start Extension Bisect' from Command Palette to try to identify the cause. --> <!-- 📣 Issues caused by an extension need to be reported directly to the extension publisher. The 'Help > Report Issue' dialog can assist with this. --> - VS Code Version: 1.70.1 - OS Version: Windows 10 Steps to Reproduce: 1. Run the command for the three-way diff: `code --disable-extensions -m --merge LOCAL.cpp REMOTE.cpp BASE.cpp OUTPUT.cpp` 2. Observe that the command returns after the window is launched.