Improve gen-release-vote.py and document

[tardieu]

gen-release-vote.py expects the refs/remotes/upstream/master branch exists in the current clone.

https://github.com/apache/openwhisk-release/blob/master/tools/gen-release-vote.py#L91

This is not the case when using a simple clone of the openwhisk-release repo.

The script could use better documentation, e.g., an example invocation. See
https://github.com/apache/openwhisk-release/blob/master/docs/release_instructions.md#initiate-a-release-vote.

[tardieu]

I think the gen-release-vote.py should simply embed the hash for the local clone, since it most likely contains the version of rcverify.sh used by the release manager.

On the other hand, the script could alert the user that the hash does not match the head of the master branch.

[alexkli]

Ran into the same issue: https://lists.apache.org/thread.html/re06acf173cbe01b225cb454de886881ff5e4e4c7946dd568da62f28f%40%3Cdev.openwhisk.apache.org%3E

Maybe make the remote name (upstream vs. origin) configurable through a cli arg and print a hint if the git show-ref returns an error, pointing to that cli arg?

./gen-release-vote.py --remote origin ...