fix(packages/core): pipeStages splits command with >> incorrectly

part of #7413

packages/core/src/repl/pipe-stages.ts

@@ -25,8 +25,14 @@ import { CommandLine } from '../models/command'
  *
  */
 export function splitIntoPipeStages(command: string): CommandLine['pipeStages'] {
+  // This covers the standard bash convention of using -- to split the
+  // command line into a "prefix" part and a "rest" part.
   const dashDashPattern = /--\s/
-  const redirectPattern = /([^"'\\])>/
+
+  // This covers > and >>, but excludes '>' and ">" and \> and 2>&1
+  // WARNING: this regexp uses negaive lookbehind. Firefox only got
+  // support for this in Firefox version 78 (released July 30 2020).
+  const redirectPattern = /(?<![\d"'\\])>>?(?![\d&])/
 
   const dashDashMatch = command.match(dashDashPattern)
   const pipeStartIdx = dashDashMatch ? dashDashMatch.index + '--'.length : undefined