Support Repositories view multi-select

[benibenj]

Copilot Generated Description: Implement multi-select functionality for SCM items, allowing users to select multiple items simultaneously.

closes #280598

[Copilot]

Pull request overview

This PR adds multi-select behavior to the SCM Repositories view by introducing selection grouping (via IIdentityProvider.getGroupId) and updating Git SCM artifact commands to handle multiple selected artifacts where appropriate.

Changes:

• Add getGroupId support to tree identity provider plumbing (async/compressed trees) and add regression tests to ensure updateOptions() preserves wrapped providers.
• Enable multi-selection in the SCM repositories tree and implement selection grouping to control which items can be multi-selected together.
• Update Git “artifact” commands (branch/tag delete, stash drop, worktree delete) to accept multiple selected artifacts.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/vs/workbench/contrib/scm/browser/scmRepositoriesViewPane.ts	Enables multi-selection in the SCM tree, adds group-based selection behavior, and wires context menu actions for artifact multi-select.
src/vs/base/test/browser/ui/tree/objectTree.test.ts	Adds regression tests ensuring updateOptions() doesn’t drop wrapped identity/accessibility providers.
src/vs/base/browser/ui/tree/compressedObjectTreeModel.ts	Ensures wrapped identity providers preserve getGroupId for compressed nodes.
src/vs/base/browser/ui/tree/asyncDataTree.ts	Ensures diffIdentityProvider mapping preserves getGroupId.
extensions/git/src/commands.ts	Updates selected Git SCM artifact commands to accept arrays and apply operations across multiple selected artifacts.