orphan/unborn: fix use of 'orphan' in end-user facing messages

orphan/unborn: fix use of 'orphan' in end-user facing messages

"orphan branch" is not even grammatical ("orphaned branch" is), and
we have been using "unborn branch" to mean the state where the HEAD
points at a branch that does not yet exist.

Update end-user facing messages to correct them.  There are cases
other random words are used (e.g., "unparented branch") but now we
have a glossary entry, use the term "unborn branch" consistently.

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