feat(vcs): show shelved changes and act on single files (#531)

The shelf could only unshelve or delete a whole changelist, and could not show what it contained: file selection lost its scope and shelf data was not connected to the diff and patch workflows.

The context menu now has Show Diff, Show Diff in a New Tab, Compare with Local, Create Patch, Copy as Patch to Clipboard and Import Patches, alongside Unshelve, Rename and Delete. Actions on a file node apply to that file only, including unshelve and delete. Diffs use the base saved with the shelf, and quoted, added, deleted and binary files are handled.

Fixes #522

src/features/vcs/commitStore.ts

@@ -29,7 +29,7 @@ interface CommitState {
   tab: CommitTab;
   /** Change list of the selected tree node (list actions without an argument). */
   selectedList: string | null;
-  /** Selected entry of the Shelf tab. */
+  /** Selected shelf tree key: shelf id, optionally followed by NUL and a file path. */
   selectedShelf: string | null;
 }