C++: Check in release that the correct author is set in git

We should check if the correct author to sign the release tarball is set as the git author. Not sure how we can detect that but I already run twice into the problem that the git author was not the same as I signed the tarball with and the release script aborted half-way. This was not catched in the dry-run mode.

**Reporter**: [Uwe Korn](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=uwe) / @xhochy

_{**Note**: *This issue was originally created as [PARQUET-912](https://issues.apache.org/jira/browse/PARQUET-912). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start by locating the release script and comparing its normal and dry-run paths, focusing on where the git author is checked before signing the tarball. Done means an incorrect author is detected before the release aborts, and dry-run mode catches the same problem.

Written by the indexing model from the issue text.