I recently encountered an error where I forgot to add a changeid locally to
some-but-not-all commits in a stack of commits. When attempting to upload
these commits via `git cl upload --no-squash`, I received a confusing message
instructing me to install the Gerrit commit-msg hook even after the hook was
installed. In order to debug this problem locally, I added the ability to
prevent `git cl upload ...` from attempting to change the contents of
messages. If the --no-add-changeid flag is passed, then the command will
simply fail quasi-informatively.

BUG=None
TEST=attempted to upload this CL via `git cl upload --no-squash --no-add-changeid`, confirm that it fails.

Change-Id: Id5f721c18143448db801095946f490880bb61786
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2828775Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Gregory Nisbet <gregorynisbet@google.com>