orphan/unborn: add to the glossary and use them consistently

orphan/unborn: add to the glossary and use them consistently

To orphan is a verb that denotes the act of getting on an unborn
branch, and a few references to "orphan branch" in our documentation
are misuses of the word.  They caused end-user confusion, which was
made even worse because we did not have the term defined in the
glossary document.  Add entries for "unborn" branch and "orphan"
operation to the glossary, and adjust existing documentation
accordingly.

Signed-off-by: Junio C Hamano <gitster@pobox.com>