llvm_tools: don't check the same SHA for reverts twice

It occurred to me that there are times where llvm-next == llvm-stable.
If that's the case, we shouldn't generate two emails with (nearly)
identical contents.

BUG=chromium:1046988
TEST=unittests

Change-Id: Ib19db46daf95e73c64cc958d57c4220fc957bb51
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2171297
Reviewed-by: Tiancong Wang <tcwang@google.com>
Tested-by: George Burgess <gbiv@chromium.org>
1 file changed