Support range select for staging/discarding files

As part of this, you must now press enter on a merge conflict file
to focus the merge view; you can no longer press space and if you do
it will raise an error.

pkg/integration/tests/worktree/worktree_in_repo.go

@@ -68,7 +68,7 @@ var WorktreeInRepo = NewIntegrationTest(NewIntegrationTestArgs{
 			Press(keys.Universal.Remove).
 			Tap(func() {
 				t.ExpectPopup().Menu().
-					Title(Equals("linked-worktree")).
+					Title(Equals("Discard changes")).
 					Select(Contains("Discard all changes")).
 					Confirm()
 			}).