llvm_tools: Add feature to specify a terminating commit message

Allow sepcifying a terminating commit message.
This will be useful to specify CL dependency information e.g.
"Cq-Depend: chromium:NNN".

BUG=chromium:1067029
TEST=unit tests

Change-Id: I2dfa751b28f64df50169cd424d39a3f3f4f1de6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2133241
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: George Burgess <gbiv@chromium.org>
4 files changed