Allow an optional supplemental commit message on builds

Change-Id: I8a9cd32b50121f6c02b78b1f97eb78d6946cd71e
Reviewed-on: https://swiftshader-review.googlesource.com/3476
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Greg Hartman <ghartman@google.com>