fix(vcs): fill the diff dialog with the editors (#3066)

The front diff dialog (the modal commit interface's Show Diff) left the area below a
short diff in the dialog's background: its body wrapper used flex: 1 inside the
dialog body, which is not a flex container, so it only took the content's height.
It now fills the body, and the editors' background reaches the footer.

src/features/vcs/diffDialog.tsx

@@ -109,7 +109,8 @@ function FrontWindow({ title, onClose, children }: { title: string; onClose: ()
       <div
         ref={body}
         className="vcsDiffDialogBody"
-        style={{ display: "flex", flexDirection: "column", flex: 1, minHeight: 0 }}
+        // The dialog body is not a flex container: fill it so the editors reach the footer.
+        style={{ display: "flex", flexDirection: "column", height: "100%", minHeight: 0 }}
       >
         {children}
       </div>