feat(vcs): compare folder selections with revisions across repositories (#602)

Compare with Branch or Revision only handled one file, so reviewing a folder meant opening each file. Folders, the project root and multiple selections now open a changes tree against the chosen revision, with per-file diffs on demand. A single file keeps its direct diff.

Each path resolves against its own repository through the root registry. In a multi-repo workspace the picker offers shared revisions such as "main in 3 repositories" as well as per-repository ones, results are grouped by repository, and content outside any repository or a repository missing the revision is stated in the result. Listing is cancellable and off the UI thread, includes worktree and untracked changes, and a rename leaving the selection shows as a delete. Show Diff on folders reuses the same tree.

Verified on macOS, and against a remote host over the server protocol. A remote window, Windows and Linux were not run.

Fixes #567

src/features/project-tree/contextMenu.ts

@@ -335,7 +335,9 @@ export function contextMenuItems(nodes: ProjectNode[]): MenuItem[] {
           { kind: "separator" },
           singleFile && act("vcs.annotate", singleFile.path, "Annotate with Git Blame"),
           act("vcs.showDiff", arg, "Show Diff"),
-          singleFile && act("vcs.compareWithBranch", { path: singleFile.path }, "Compare with Branch…"),
+          nodes.length > 0 &&
+            nodes.every((n) => !n.placeholder) &&
+            act("vcs.compareWithBranch", arg, "Compare with Branch…"),
           nodes.length > 0 && nodes.every((n) => !n.placeholder) && act("vcs.showHistory", arg, "Show History"),
           act("vcs.rollback", arg, "Rollback…"),
           { kind: "separator" },