Git 1.9.1

The version numbering scheme has changed since Git 1.9 and we
dropped the third dewey-decimal from the traditional numbering
(e.g. both 1.8.4 and 1.8.5 were major feature releases).  This
release 1.9.1 is the first maintenance relase for Git 1.9.

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