llvm_tool: support differential reviews in the cherry-pick tool.

Rename cherrypick_cl.py to get_llvm_upstream.py, and add --differential
option for creating local patches based on differential reviews. This
should help testing experimental Clang/LLVM patches on ChromeOS.

BUG=chromium:1202311
TEST=Successfully run the script with --sha $SHA --differential
$DIFFERENTIAL_REVISION

Change-Id: I7bd1704d1079352db52fc4690ef453436fe854de
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2861111
Tested-by: Jian Cai <jiancai@google.com>
Reviewed-by: George Burgess <gbiv@chromium.org>
4 files changed