llvm_tools: rename extra_commit_msg for clarity

`extra_commit_msg` is somewhat ambiguous as to what it is; adding
`_lines` makes it clearer that an iterable of lines is expected.

BUG=b:301250218
TEST=Unittests

Change-Id: I757bda6ceafe4514b76d85e85108c15e3f1c32eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/4877352
Tested-by: George Burgess <gbiv@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Reviewed-by: Jordan Abrahams-Whitehead <ajordanr@google.com>
4 files changed