completion: introduce __gitcomp_nl_append ()

There are situations where multiple classes of completions possible. For
example

branch.<TAB>

should try to complete

branch.master.
branch.autosetupmerge
branch.autosetuprebase

The first candidate has the suffix ".", and the second/ third candidates
have the suffix " ". To facilitate completions of this kind, create a
variation of __gitcomp_nl () that appends to the existing list of
completion candidates, COMPREPLY.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>