bash completion: Don't offer "a.." as a completion for "a."

If the user is trying to complete "v1.5.3.<tab>" to see all of
the available maintenance releases for 1.5.3 we should not give
them an extra dot as the completion.  Instead if the user wants
a ".." or a "..." operator they should key the two dots out on
their own.  Its the same number of keystrokes either way.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>