Annotate a warning/error if unshelved changelist contents differed in any steps

[ca-johnson]

hash = md5()
for digest in changeinfo['digests']:
  md5.add(digest)
if not set_metadata('buildkite:perforce:changedigest', md5.hex()'):
  # Already set, compare against
  if get_metadata(key) != md5.hex():
	buildkite-agent annotate --type error "Shelved files were modified during the build, some steps were ran against different versions of your shelved files. Please run another build to validate your change prior to submission"