[MRELEASE-644] Do a pre-check for Git repositories to make sure the name and email are set

**[Jason van Zyl](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=jvanzyl)** opened **[MRELEASE-644](https://issues.apache.org/jira/browse/MRELEASE-644?redirect=false)** and commented

I went to do a release on a new machine and forgot to run:

git config --global user.email "you@example.com"
git config --global user.name "Your Name"

And it got to the very end of release:prepare and blew up.

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No further details from [MRELEASE-644](https://issues.apache.org/jira/browse/MRELEASE-644?redirect=false)

Start at the release:prepare flow and inspect how it interacts with the Git repository. Check how Git reports missing user.name and user.email, then define completion as detecting either missing setting before the release reaches its final stages and reporting an actionable message.

Written by the indexing model from the issue text.