llvm_tools: add commit subjects to revert emails

I got a piece of feedback that adding subjects inline would likely make
revert emails easier to read 'at a glance'. This CL adds those.

New email revert lines will look like:
- r385979 (appears to revert r383879): Revert "[NFC][ARM] Update test"

BUG=None
TEST=unittests; looked at a --dry_run email

Change-Id: I5fb5ea12fa1d4a6b2601fbd5c244935662dc4270
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2174823
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
2 files changed