The --tag TAG option is now printing the changes for the given TAG and
not since the given tag. The new option --galaxy is printing the changelog
since the latest tag and also for the latest tag.

These changes are simplifying the generation of the changelog file that
is needed to pass the tests for galaxy and AutomationHub collections.

The function get_commit was using the global merge variable instead of
the local commit variable. Therefore it returned the wrong commit
subject for merges without subject.

There is curently a merge commit without a subject, which leads into a
traceback in the changelog script.

The merge information provides the commit hash, which is now used to get
the subject later on using the generated commits hash.

This scrip can be used to generate the changelog text for a new
ansible-freeipa release on github.

    usage: Usage: changelog [options] [<new version>]

    optional arguments:
      -h, --help  show this help message and exit
      --tag TAG   git tag

If the script is used without a givn tag, it will show all the changes since
the last tag. If a tag (this can be a also a commit) is given, then all
changes since this commit are shown.