buildAndPushRelease.py should ensure you have no unpushed git changes [LUCENE-7021]

I'm just a git newbie so I'm not sure how to add this to our release scripts, but I think it's quite important that we don't up and release something the world containing changes I had only committed locally and then failed to push. What if I then remove that directory after we've released?

We should also fix the release smoke tester to confirm the git revision is known ... I'll open a separate issue for that.

I'm trying to be super careful not to do this for 5.5.0, but really the release script should catch this.

I did a bit of googling and found the magic command `git cherry -v origin/branch_5_5` seemed to work (at least, it showed my one local commit), but e.g. "origin" is just a label I use (others use "upstream"), etc., so I'm not sure how to reliably do this... maybe we have to run `git remote -v` and figure out what label use use for the "official" (wip) Apache git instance?

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Migrated from [LUCENE-7021](https://issues.apache.org/jira/browse/LUCENE-7021) by Michael McCandless (@mikemccand)

Start by reading buildAndPushRelease.py and the surrounding release scripts, then inspect how git cherry and git remote -v behave with different remote labels. Done means the release process reliably detects unpushed local changes before publishing, while the separate release smoke-tester request remains out of scope.

Written by the indexing model from the issue text.