Add a --brief option to git_changelog.

In commit c0b050192, Andres introduced the idea of including one-line
commit references in our major release notes.  Teach git_changelog to
emit a (lightly adapted) version of that format, so that we don't
have to laboriously add it to the notes after the fact.  The default
output isn't changed, since I anticipate still using that for minor
release notes.

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