refactor: use non-deprecated indexmap methods

The `remove()` and `take()` methods are deprecated in favor of
`swap_remove()`, `shift_remove()`, `swap_take()`, and `shift_take()`,
which have been available since indexmap 1.2.0.

The "shift" variants are used in StGit because they will preserve set
order at the cost of being O(n). But since parent and patch sets are not
particularly large, perhaps not a big concern.

src/stack/state.rs

@@ -253,7 +253,7 @@ impl<'repo> StackState<'repo> {
             parent_set.insert(prev_commit.id);
             let prev_state = prev_state.as_ref().unwrap();
             for patchname in prev_state.all_patches() {
-                parent_set.remove(&prev_state.patches[patchname].commit.id);
+                parent_set.shift_remove(&prev_state.patches[patchname].commit.id);
             }
         }
 

src/stack/transaction/mod.rs

@@ -631,7 +631,7 @@ impl<'repo> StackTransaction<'repo> {
         self.hidden = hidden;
 
         for pn in self.all_patches() {
-            old.take(pn)
+            old.shift_take(pn)
                 .expect("new patchname must be an old patchname");
         }
         assert!(