Don't overwrite passed kwargs. Make a copy instead.
Otherwise, subsequent calls with the same kwargs will have
stdin set to subprocess.PIPE.

Bug: 984182
Change-Id: I358ffa1951e8b42486e0ac3a0d3d587a93c6dc4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1769405
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>