fix(vcs): cap the Push dialog's height so a long commit list scrolls inside it (#1491)

With many outgoing commits the Push dialog grew to the full screen height and its
whole body scrolled besides the changed-files tree, because the body was not a flex
container and the commit list's flex had no effect. The single- and multi-repository
Push dialogs now open at most 720px tall with the commit list filling the rest and
scrolling on its own; Dialog gets a maxHeight that applies until the user resizes.

Closes #1485

Co-authored-by: jaw52 <[email protected]>

src/ui/Dialog.test.tsx

@@ -168,6 +168,21 @@ describe("Dialog chrome", () => {
     expect([dialog.style.width, dialog.style.height]).toEqual(["600px", "400px"]);
   });
 
+  it("caps a content-sized dialog at maxHeight until it is resized", () => {
+    render(
+      <Dialog open onClose={() => {}} title="Push" resizable="both" maxHeight={720}>
+        <div>rows</div>
+      </Dialog>,
+    );
+    const dialog = screen.getByRole("dialog");
+    expect(dialog.style.maxHeight).toBe("min(720px, 100%)");
+    fireEvent.pointerDown(screen.getByRole("separator", { name: "Resize" }), { button: 0, clientX: 0, clientY: 0 });
+    fireEvent.pointerMove(window, { buttons: 1, clientX: 500, clientY: 800 });
+    fireEvent.pointerUp(window);
+    expect(dialog.style.maxHeight).toBe("");
+    expect(dialog.style.height).not.toBe("");
+  });
+
   it("still drags a window dialog by its title bar", () => {
     render(
       <Dialog open onClose={() => {}} title="Usages Detected" buttons={safeDelete()} platform="other">